Provide configurable fallback image for failed tile sources

[marlo-longley]

See #4207

• Allow config.fallbackImage to be set, optionally.
• Provide a default fallbackImage that can be used if the setting is uncommented.

This is more of a proposal and will need discussion about design and implementation. But it does work!

Using provided default config.fallbackImage (this could use designer input):

With config.fallbackImage unset (improvement: maintains thumbnail size as requested in #4207):

This also works in the gallery view:

[codecov]

Codecov Report

❌ Patch coverage is 89.15663% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.1%. Comparing base (5af3981) to head (cbcefb8).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/components/OpenSeadragonTileSource.jsx	74.3%	9 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #4211     +/-   ##
=======================================
- Coverage   95.1%   95.1%   -0.0%     
=======================================
  Files        326     328      +2     
  Lines      16192   16252     +60     
  Branches    2546    2560     +14     
=======================================
+ Hits       15391   15442     +51     
- Misses       796     805      +9     
  Partials       5       5             

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