Better error message if attempt fancy indexing

[alimanfoo]

Currently if pass boolean array, get:

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-24-06bb9af56726> in <module>()
----> 1 ann_vgsc_gene = callset['2L/variants/ANN'][loc1]
      2 ann_vgsc_gene

/home/aliman/miniconda3/envs/biipy240/lib/python3.5/site-packages/zarr/core.py in __getitem__(self, item)
    446 
    447         # normalize selection
--> 448         selection = normalize_array_selection(item, self._shape)
    449 
    450         # determine output array shape

/home/aliman/miniconda3/envs/biipy240/lib/python3.5/site-packages/zarr/util.py in normalize_array_selection(item, shape)
    176     elif isinstance(item, slice):
    177         item = (item,)
--> 178     elif item == Ellipsis:
    179         item = (slice(None),)
    180 

ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()