Adapting log level when files are not found

[sebastienros]

Fixes #143

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Description

Error is too intrusive and these events are too common, the event log can then fill up quickly. Moving it to Debug since this is what aspnet uses for the standard static file provider on these events.

[codecov]

Codecov Report

Merging #148 (9908d55) into master (7ab73bb) will decrease coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #148      +/-   ##
==========================================
- Coverage   84.64%   84.57%   -0.08%     
==========================================
  Files          46       46              
  Lines        1368     1368              
  Branches      177      177              
==========================================
- Hits         1158     1157       -1     
- Misses        162      163       +1     
  Partials       48       48              

Flag	Coverage Δ
unittests	84.57% <100.00%> (-0.08%)	⬇️

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Impacted Files	Coverage Δ
src/ImageSharp.Web/Middleware/LoggerExtensions.cs	87.09% <100.00%> (ø)
.../ImageSharp.Web/Middleware/ImageSharpMiddleware.cs	87.87% <0.00%> (-0.38%)	⬇️

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[JimBobSquarePants]

Perfect. Thanks!