Better error message for setting encoding["units"]

[yt87]

What happened:

Setting invalid units for time axis encoding results in an exception AttributeError: 'NoneType' object has no attribute 'groups'

What you expected to happen:

It should say "invalid time units", like this (see commented out line below) ValueError: invalid time units: days after 1/3/2000

Minimal Complete Verifiable Example:

# Put your MCVE code here
import pandas as pd
import xarray as xr
ds = xr.Dataset(data_vars={'v': (('t',), [0,])},
                coords={'t': [pd.Timestamp(2000, 1, 1)]})
ds.t.encoding['units'] = 'days since Big Bang'
#ds.t.encoding['units'] = 'days after 1/3/2000'
ds.to_netcdf('/tmp/x.nc', mode='w')

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-16-0d97a797be64> in <module>
      5 ds.t.encoding['units'] = 'days since Big Bang'
      6 #ds.t.encoding['units'] = 'days after 1/3/2000'
----> 7 ds.to_netcdf('/tmp/x.nc', mode='w')

~/miniconda3/envs/xarray/lib/python3.9/site-packages/xarray-0.0.0-py3.9.egg/xarray/core/dataset.py in to_netcdf(self, path, mode, format, group, engine, encoding, unlimited_dims, compute, invalid_netcdf)
   1752         from ..backends.api import to_netcdf
   1753 
-> 1754         return to_netcdf(
   1755             self,
   1756             path,

~/miniconda3/envs/xarray/lib/python3.9/site-packages/xarray-0.0.0-py3.9.egg/xarray/backends/api.py in to_netcdf(dataset, path_or_file, mode, format, group, engine, encoding, unlimited_dims, compute, multifile, invalid_netcdf)
   1066         # TODO: allow this work (setting up the file for writing array data)
   1067         # to be parallelized with dask
-> 1068         dump_to_store(
   1069             dataset, store, writer, encoding=encoding, unlimited_dims=unlimited_dims
   1070         )

~/miniconda3/envs/xarray/lib/python3.9/site-packages/xarray-0.0.0-py3.9.egg/xarray/backends/api.py in dump_to_store(dataset, store, writer, encoder, encoding, unlimited_dims)
   1113         variables, attrs = encoder(variables, attrs)
   1114 
-> 1115     store.store(variables, attrs, check_encoding, writer, unlimited_dims=unlimited_dims)
   1116 
   1117 

~/miniconda3/envs/xarray/lib/python3.9/site-packages/xarray-0.0.0-py3.9.egg/xarray/backends/common.py in store(self, variables, attributes, check_encoding_set, writer, unlimited_dims)
    261             writer = ArrayWriter()
    262 
--> 263         variables, attributes = self.encode(variables, attributes)
    264 
    265         self.set_attributes(attributes)

~/miniconda3/envs/xarray/lib/python3.9/site-packages/xarray-0.0.0-py3.9.egg/xarray/backends/common.py in encode(self, variables, attributes)
    350         # All NetCDF files get CF encoded by default, without this attempting
    351         # to write times, for example, would fail.
--> 352         variables, attributes = cf_encoder(variables, attributes)
    353         variables = {k: self.encode_variable(v) for k, v in variables.items()}
    354         attributes = {k: self.encode_attribute(v) for k, v in attributes.items()}

~/miniconda3/envs/xarray/lib/python3.9/site-packages/xarray-0.0.0-py3.9.egg/xarray/conventions.py in cf_encoder(variables, attributes)
    841     _update_bounds_encoding(variables)
    842 
--> 843     new_vars = {k: encode_cf_variable(v, name=k) for k, v in variables.items()}
    844 
    845     # Remove attrs from bounds variables (issue #2921)

~/miniconda3/envs/xarray/lib/python3.9/site-packages/xarray-0.0.0-py3.9.egg/xarray/conventions.py in <dictcomp>(.0)
    841     _update_bounds_encoding(variables)
    842 
--> 843     new_vars = {k: encode_cf_variable(v, name=k) for k, v in variables.items()}
    844 
    845     # Remove attrs from bounds variables (issue #2921)

~/miniconda3/envs/xarray/lib/python3.9/site-packages/xarray-0.0.0-py3.9.egg/xarray/conventions.py in encode_cf_variable(var, needs_copy, name)
    267         variables.UnsignedIntegerCoder(),
    268     ]:
--> 269         var = coder.encode(var, name=name)
    270 
    271     # TODO(shoyer): convert all of these to use coders, too:

~/miniconda3/envs/xarray/lib/python3.9/site-packages/xarray-0.0.0-py3.9.egg/xarray/coding/times.py in encode(self, variable, name)
    510             variable
    511         ):
--> 512             (data, units, calendar) = encode_cf_datetime(
    513                 data, encoding.pop("units", None), encoding.pop("calendar", None)
    514             )

~/miniconda3/envs/xarray/lib/python3.9/site-packages/xarray-0.0.0-py3.9.egg/xarray/coding/times.py in encode_cf_datetime(dates, units, calendar)
    448         units = infer_datetime_units(dates)
    449     else:
--> 450         units = _cleanup_netcdf_time_units(units)
    451 
    452     if calendar is None:

~/miniconda3/envs/xarray/lib/python3.9/site-packages/xarray-0.0.0-py3.9.egg/xarray/coding/times.py in _cleanup_netcdf_time_units(units)
    399 
    400 def _cleanup_netcdf_time_units(units):
--> 401     delta, ref_date = _unpack_netcdf_time_units(units)
    402     try:
    403         units = "{} since {}".format(delta, format_timestamp(ref_date))

~/miniconda3/envs/xarray/lib/python3.9/site-packages/xarray-0.0.0-py3.9.egg/xarray/coding/times.py in _unpack_netcdf_time_units(units)
    130 
    131     delta_units, ref_date = [s.strip() for s in matches.groups()]
--> 132     ref_date = _ensure_padded_year(ref_date)
    133 
    134     return delta_units, ref_date

~/miniconda3/envs/xarray/lib/python3.9/site-packages/xarray-0.0.0-py3.9.egg/xarray/coding/times.py in _ensure_padded_year(ref_date)
    105     # appropriately
    106     matches_start_digits = re.match(r"(\d+)(.*)", ref_date)
--> 107     ref_year, everything_else = [s for s in matches_start_digits.groups()]
    108     ref_date_padded = "{:04d}{}".format(int(ref_year), everything_else)
    109 

AttributeError: 'NoneType' object has no attribute 'groups'

Anything else we need to know?:
Are there detail specifications for the valid units string? Setting it to 1/3/2000 surprised me (my locale: LANG=en_CA.UTF-8).
Environment:
Latest code from github: xarray version 0.0.0

Output of xr.show_versions()

INSTALLED VERSIONS ------------------ commit: None python: 3.9.2 | packaged by conda-forge | (default, Feb 21 2021, 05:02:46) [GCC 9.3.0] python-bits: 64 OS: Linux OS-release: 5.10.11-200.fc33.x86_64 machine: x86_64 processor: x86_64 byteorder: little LC_ALL: None LANG: en_CA.UTF-8 LOCALE: en_CA.UTF-8 libhdf5: 1.10.6 libnetcdf: 4.8.0

xarray: 0.0.0
pandas: 1.2.4
numpy: 1.20.2
scipy: None
netCDF4: 1.5.6
pydap: None
h5netcdf: None
h5py: None
Nio: None
zarr: 2.7.1
cftime: 1.4.1
nc_time_axis: None
PseudoNetCDF: None
rasterio: None
cfgrib: None
iris: None
bottleneck: None
dask: 2021.04.0
distributed: 2021.04.0
matplotlib: None
cartopy: None
seaborn: None
numbagg: None
pint: None
setuptools: 49.6.0.post20210108
pip: 21.0.1
conda: None
pytest: None
IPython: 7.22.0
sphinx: None