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.