Minor UX improvement: Add the UnknownCodecError.

[cwognum]

See zarr-developers/zarr-python#2508 for context.

Since numcodecs supports registering custom codecs in downstream libraries, there can be situations where the environment in which data was encoded is not the same as the environment in which the data is decoded (e.g. opening a Zarr archive that was created in a different environment). This can lead to missing codecs.

It would be a nice UX improvement if you catch that error and can offer codec specific solutions.

This change would enable something like:

import numcodecs

try: 
    numcodecs.get_codec({"codec_id": "imagecodecs_jpeg2k"})
except UnknownCodecError as error:
    if error.codec_id == "imagecodecs_jpeg2k":
        raise RuntimeError("This Zarr archive requires `imagecodecs`, to install it use ...") from error

TODO:

• Unit tests and/or doctests in docstrings
• Tests pass locally
• Docstrings and API docs for any new/modified user-facing classes and functions
• Changes documented in docs/release.rst
• Docs build locally
• GitHub Actions CI passes
• Test coverage to 100% (Codecov passes)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.70%. Comparing base (7ed1ace) to head (e4dac91).
Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #689   +/-   ##
=======================================
  Coverage   99.70%   99.70%           
=======================================
  Files          62       63    +1     
  Lines        2749     2755    +6     
=======================================
+ Hits         2741     2747    +6     
  Misses          8        8           

Files with missing lines	Coverage Δ
numcodecs/errors.py	100.00% <100.00%> (ø)
numcodecs/registry.py	100.00% <100.00%> (ø)
numcodecs/tests/test_registry.py	100.00% <100.00%> (ø)

[dstansby]

I'm very 👍 doing this, thanks for the PR. I left a suggestion for the test, and also I'm a bit on the fence about ValueError, so let me know your thoughts there.

[cwognum]

@d-v-b @dstansby Ready for another round of review!