Add informative error message

[hungyiwu]

Given the current error message, I figured a more informative error message will help me a lot, and perhaps other people like me as well.
#261 (comment)

See #261 for more detail.

[Description of PR]

TODO:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/tutorial.rst
• Changes documented in docs/release.rst
• AppVeyor and Travis CI passes
• Test coverage is 100% (Coveralls passes)

[pep8speaks]

Hello @hungyiwu! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file zarr/convenience.py:

Line 76:101: E501 line too long (121 > 100 characters)
Line 77:24: E128 continuation line under-indented for visual indent
Line 77:101: E501 line too long (116 > 100 characters)
Line 78:1: W293 blank line contains whitespace

[hungyiwu]

See #261 (comment)