fix: report valid color lookup values clearly

[RoboX2020]

Addresses the color.py lookup-message part of #115.

The white-point and primary-name lookups now catch KeyError rather than every exception. Their ValueError messages list the supported names with commas, instead of using the entire sentence as a separator. The original KeyError remains available as the cause.

Added two regression tests checking the exact messages and exception causes. Both tests fail against the original implementation.

Validation on Python 3.11 / OpenCV 5.0.0.93:

• Full pytest suite: 961 passed, 97 skipped, 5 subtests passed.
• Color tests: 14 passed, 1 skipped.
• New test file passes ruff and black; changed source passes black.

OpenCV 4 was not run for this PR. The other exception sites in #115 are left unchanged.

[codacy-production]

Up to standards ✅

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[petercorke]

Welcome to MVTB! Thanks for your contributions here and in RTB and bdsim. This is a lovely contribution: the old ", ".join(...) was using the whole sentence as the separator, which made the error message nonsense, and your tests pin the exact wording and the exception cause. Merged, and thanks for taking the time, it's much appreciated.