[FIX] io.opus: Convert visible image loading Exceptions to warnings

[stuart-cls]

This makes a whole class of opusFC bugs less annoying to users.

Also today I learned that warnings are nicely propagated up to the File widget!

[codecov-commenter]

Codecov Report

Merging #653 (6034d84) into master (6b47128) will decrease coverage by 0.07%. The diff coverage is 60.00%.

@@            Coverage Diff             @@
##           master     #653      +/-   ##
==========================================
- Coverage   89.45%   89.38%   -0.08%     
==========================================
  Files          71       71              
  Lines       11842    11847       +5     
==========================================
- Hits        10593    10589       -4     
- Misses       1249     1258       +9     

[markotoplak]

Thanks!