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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