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 IMedia

Edited Aug 08, 2019 by Hugo Beauzée-Luyssen

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