Make list_chunkmanagers more resilient to broken entrypoints

[hmaarrfk]

As I'm a developing my custom chunk manager, I'm often checking out between my development branch and production branch breaking the entrypoint.

This made xarray impossible to import unless I re-ran pip install -e . -vv which is somewhat tiring.

This should help xarray be more resilient in other software's bugs in case they install malformed entrypoints

Example:

>>> from xarray.core.parallelcompat import list_chunkmanagers

>>> list_chunkmanagers()
<ipython-input-3-19326f4950bc>:1: UserWarning: Failed to load entrypoint MyChunkManager due to No module named 'my.array._chunkmanager'. Skipping.
  list_chunkmanagers()
{'dask': <xarray.core.daskmanager.DaskManager at 0x7f5b826231c0>}

Thank you for considering.

• Closes #xxxx
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

This is mostly a quality of life thing for developers, I don't see this as a user visible change.

[TomNicholas]

This looks great! Is it possible to test this error message somehow?

[hmaarrfk]

and rebased. please see suggested test.

[TomNicholas]

Amazing! I've just pushed a commit to try to appease mypy.

[TomNicholas]

You can also put an entry in what's new if you want :)

[TomNicholas]

hmm - maybe adding -> None to utils.emit_user_level_warning will work?

[hmaarrfk]

You can also put an entry in what's new if you want :

let’s keep the scope of this fix small. I don’t think it warrants much of a message.

let’s keep the message for when we tackle the underlying challenge that brought me here ^_^