Let caller hander error cases

[michalpristas]

Please consider this.
This PR introduces onErr option in a form of func(error)error which lets caller to handler error cases .
It enables caller to log error by error and gives a possibility to continue with copying in case error is not that severe by returning nil (swallowing an error).

[codecov]

Codecov Report

Merging #101 (b348b50) into main (dfe4363) will increase coverage by 1.03%.
The diff coverage is 100.00%.

❗ Current head b348b50 differs from pull request most recent head 2abaf6c. Consider uploading reports for the commit 2abaf6c to get more accurate results

@@            Coverage Diff             @@
##             main     #101      +/-   ##
==========================================
+ Coverage   78.33%   79.36%   +1.03%     
==========================================
  Files           9        9              
  Lines         180      189       +9     
==========================================
+ Hits          141      150       +9     
  Misses         19       19              
  Partials       20       20              

Impacted Files	Coverage Δ
options.go	83.33% <ø> (ø)
copy.go	80.32% <100.00%> (+1.56%)	⬆️

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[otiai10]

in case error is not that severe

Can you give me some example of this?

[michalpristas]

what i'm facing is a case when (on windows) directory copy fails because another process is reading a single file.
what i'd like to do here is to have a hook so I can decide on severity, and skip file if it's not critical for me

[otiai10]

@otiai10 will check in 24 hours