fetchart: handle FilesystemError when setting album art

[lawrence3699]

Description

Fixes #6193.

Problem

When fetchart encounters a FilesystemError during set_art (e.g. a file locked by another process on Windows, or a cross-device move permission error), the unhandled exception crashes the import session or the fetchart CLI command.

The cleanup method at line ~520 already catches FilesystemError and logs it gracefully, but the two _set_art call sites don't.

Fix

Wrap both _set_art call sites in try/except util.FilesystemError:

• assign_art() (import pipeline): log the error and return early, skipping the prune step since no art was set.
• batch_fetch_art() (CLI command): log the error and show "error setting art" in the output instead of crashing.

Tests

Added two tests covering both error paths:

• test_batch_fetch_filesystem_error — verifies the CLI command reports the error message instead of crashing.
• test_assign_art_filesystem_error — verifies the import path returns gracefully on error.

All existing fetchart tests pass.

To Do

• Changelog
• Tests
• Documentation

[github-actions]

Thank you for the PR! The changelog has not been updated, so here is a friendly reminder to check if you need to add an entry.

[Copilot]

Pull request overview

PR make fetchart not crash when filesystem say “no” during set_art (Windows lock, cross-device move, etc). PR fit in beetsplug fetchart reliability: keep import + CLI running, just warn and move on.

Changes:

• Catch util.FilesystemError around _set_art in import hook (assign_art) and CLI path (batch_fetch_art).
• On CLI error, print “error setting art” instead of throwing.
• Add tests for both error paths.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
beetsplug/fetchart.py	Wrap _set_art calls with try/except util.FilesystemError and log + continue/return.
test/plugins/test_fetchart.py	Add regression tests for FilesystemError in CLI and import assign path.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.42%. Comparing base (fd586ef) to head (b03b5c3).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6516      +/-   ##
==========================================
+ Coverage   70.40%   70.42%   +0.02%     
==========================================
  Files         148      148              
  Lines       18806    18814       +8     
  Branches     3067     3067              
==========================================
+ Hits        13240    13250      +10     
+ Misses       4916     4913       -3     
- Partials      650      651       +1     

Files with missing lines	Coverage Δ
beetsplug/fetchart.py	74.52% <100.00%> (+0.28%)	⬆️

... and 1 file with indirect coverage changes

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[snejus]

Looks good. Just fix the formatting issues and add a note to the changelog please.