Allow runtime physical deletion of cached images

[JimBobSquarePants]

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Description

Allows the runtime deletion of cached images by handling any exceptions thrown during resolving the cached image. Fix #239

If an exception is thrown we do a complete run through of the processing again to ensure that any changes to both the source and image are captured. If an issue still occurs we'll log and rethrow.

CC @ronaldbarendse

[ronaldbarendse]

Looks good to me!

[codecov]

Codecov Report

Merging #240 (8bfe51f) into main (21a6bc1) will decrease coverage by 0%.
The diff coverage is 56%.

@@         Coverage Diff         @@
##           main   #240   +/-   ##
===================================
- Coverage    85%    85%   -1%     
===================================
  Files        71     71           
  Lines      1961   1969    +8     
  Branches    290    291    +1     
===================================
+ Hits       1682   1684    +2     
- Misses      201    207    +6     
  Partials     78     78           

Flag	Coverage Δ
unittests	85% <56%> (-1%)	⬇️

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Impacted Files	Coverage Δ
src/ImageSharp.Web/Middleware/ImageContext.cs	80% <ø> (ø)
.../ImageSharp.Web/Middleware/ImageSharpMiddleware.cs	77% <56%> (-3%)	⬇️

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