Improve error messages from C

[akx]

Inspired by #6667 (comment)

• New internal exception type NotSupportedError for operations that are currently not supported; derives from both ValueError and NotImplementedError for backward compatibility.
  • Deriving from NotImplementedError allows for try-excepting in tests, so we can do better than (e.g.) Tests.test_image_filter.test_sanity where the test just quietly does nothing when a mode combo is not (yet, or currently) supported. (This wasn't wired up here yet.)
• The internal ImagingError_*() C APIs now all support an optional message text, and message texts were added where practical. No longer "image has wrong mode" or "images do not match" for everything! ✨
• Image nullness checks used to occasionally do ModeErrors, occasionally ValueErrors; now they're all ValueError.

[akx]

Rebased post #9794.

[codspeed-hq]

Merging this PR will improve performance by 10.94%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 10 improved benchmarks
✅ 593 untouched benchmarks
⏩ 335 skipped benchmarks¹

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	test_scale[1237x811-I-2.14-NEAREST]	101.8 ms	91 ms	+11.86%
⚡	test_scale[1237x811-RGB-2.14-NEAREST]	101.9 ms	91.1 ms	+11.86%
⚡	test_scale[1237x811-LA-2.14-NEAREST]	101.9 ms	91.1 ms	+11.85%
⚡	test_scale[1237x811-RGBA-2.14-NEAREST]	101.9 ms	91.1 ms	+11.84%
⚡	test_scale[1237x811-F-2.14-NEAREST]	101.9 ms	91.1 ms	+11.77%
⚡	test_scale[1237x811-F-0.8-NEAREST]	16.5 ms	15 ms	+10.05%
⚡	test_scale[1237x811-LA-0.8-NEAREST]	16.5 ms	15 ms	+10.05%
⚡	test_scale[1237x811-RGB-0.8-NEAREST]	16.5 ms	15 ms	+10.05%
⚡	test_scale[1237x811-RGBA-0.8-NEAREST]	16.5 ms	15 ms	+10.05%
⚡	test_scale[1237x811-I-0.8-NEAREST]	16.5 ms	15 ms	+10.05%

Tip

Curious why performance improved? Comment @codspeedbot explain why performance improved on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing akx:wrong-mode-exc (11dc2fc) with main (0a61c53)}

Footnotes

1. 335 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩