Allow thumbnail of a specific media to be regenerated
it's currently impossible to force the regeneration of a thumbnail for a specific media. Either all will be regenerated, or nothing. However, we probably should disallow this when the previous generation failed due to a crash.
- Store the failure reason ("generic failure"/crash)
- Expose it
Add a way to discard the previous failure, either by an additional parameter to requestThumbnail, or a separate call into