fetchart: catch FilesystemError in _set_art instead of crashing

[jesustorres-code]

Fixes #6193

Problem

When fetchart tries to save artwork and the destination file is locked by another process (e.g. foobar2000 on Windows), a FilesystemError propagates uncaught from _set_art → album.set_art() → util.move(), crashing the entire import pipeline with an unhelpful traceback.

Fix

Wrap album.set_art() in a try/except util.FilesystemError block in _set_art(). On failure, log a warning with the album name and error message, then return early so the rest of the import continues.

This also covers the manual beet fetchart command path, which goes through the same _set_art() method.

Behaviour change

Before: import crashes with an unhandled exception.
After: a warning is logged ("fetchart: could not save artwork for ...") and the import continues.

[github-actions]

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[codecov]

Codecov Report

❌ Patch coverage is 40.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.45%. Comparing base (c9df7a9) to head (35b3d95).

Files with missing lines	Patch %	Lines
beetsplug/fetchart.py	40.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6660      +/-   ##
==========================================
- Coverage   72.46%   72.45%   -0.01%     
==========================================
  Files         161      161              
  Lines       20710    20714       +4     
  Branches     3276     3276              
==========================================
+ Hits        15007    15008       +1     
- Misses       4977     4980       +3     
  Partials      726      726              

Files with missing lines	Coverage Δ
beetsplug/fetchart.py	74.00% <40.00%> (-0.27%)	⬇️

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[semohr]

The import will also error out and not finished if the file is locked and can't be moved, or am I missing something? The same error would also be raised for all metadata beets wants to write to the files in the last step during the import.

This proposed fix will just postpone the error and it will be raised a few seconds later again, right?