bug: handle bad image file

[parano]

What does this PR address?

Fixing an issue first reported here in the slack community:
https://bentoml.slack.com/archives/CKRANBHPH/p1661185794381889

For input=Image(), if the uploaded file is not an image, the server should return 400 code with an error message on bad input instead of 500 indicating an issue with the user code.

Fixes #(issue)

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[codecov]

Codecov Report

Merging #2942 (b00ac1a) into main (0354edb) will decrease coverage by 0.01%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##             main    #2942      +/-   ##
==========================================
- Coverage   70.56%   70.55%   -0.02%     
==========================================
  Files         104      104              
  Lines        9496     9499       +3     
==========================================
+ Hits         6701     6702       +1     
- Misses       2795     2797       +2     

Impacted Files	Coverage Δ
bentoml/_internal/io_descriptors/image.py	83.33% <50.00%> (-2.00%)	⬇️