fetch: add basic onerror

[efiop]

Related https://github.com/iterative/studio/pull/6541

[codecov]

Codecov Report

Patch coverage: 50.00% and project coverage change: -0.32% ⚠️

Comparison is base (d553334) 90.45% compared to head (14ef36c) 90.14%.

❗ Current head 14ef36c differs from pull request most recent head 907b3b3. Consider uploading reports for the commit 907b3b3 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9810      +/-   ##
==========================================
- Coverage   90.45%   90.14%   -0.32%     
==========================================
  Files         483      483              
  Lines       36863    36868       +5     
  Branches     5321     4362     -959     
==========================================
- Hits        33344    33233     -111     
- Misses       2910     2997      +87     
- Partials      609      638      +29     

Files Changed	Coverage Δ
dvc/repo/fetch.py	79.31% <50.00%> (-2.40%)	⬇️

... and 27 files with indirect coverage changes

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