Fix the non-writable path deletion error

[vkaidalov]

Hi everyone!

I noticed that I got the "Delete Failed: Unhandled error" message in the error dialog when I tried to delete either a read-only file or a read-only directory. In contrast, the message was "Permission denied" in JupyterLab 1.x, so I decided to take a look at the code.

As the result, I found out that this issue had already been identified in the comments. Given that the "easier to ask for forgiveness than permission" approach cannot be used there because of send2trash's errors ambiguity, I propose to "look before you leap" to fix this issue.

Could you please review the PR? It should make the server return 403 instead of 500 and also make JupyterLab show the "Permission denied" message instead of the "Unhandled error" text. I've also moved the permissions check code to a separate new method.

[welcome]

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[codecov-commenter]

Codecov Report

Merging #670 (6a5bf05) into main (db46446) will increase coverage by 0.49%.
The diff coverage is 53.33%.

@@            Coverage Diff             @@
##             main     #670      +/-   ##
==========================================
+ Coverage   77.88%   78.38%   +0.49%     
==========================================
  Files         110      110              
  Lines       10405    10696     +291     
  Branches     1400     1441      +41     
==========================================
+ Hits         8104     8384     +280     
- Misses       1911     1916       +5     
- Partials      390      396       +6     

Impacted Files	Coverage Δ
jupyter_server/services/contents/filemanager.py	70.22% <53.33%> (-0.34%)	⬇️
jupyter_server/pytest_plugin.py	84.89% <0.00%> (-0.09%)	⬇️
jupyter_server/tests/services/sessions/test_api.py	98.10% <0.00%> (-0.06%)	⬇️
jupyter_server/tests/services/kernels/test_cull.py	100.00% <0.00%> (ø)
jupyter_server/tests/services/kernels/test_api.py	96.22% <0.00%> (+0.22%)	⬆️
jupyter_server/utils.py	66.47% <0.00%> (+0.56%)	⬆️
jupyter_server/nbconvert/handlers.py	25.00% <0.00%> (+0.72%)	⬆️

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[blink1073]

LGTM, thank you!

[blink1073]

Kicking CI

[Zsailer]

Kicking CI one more time after #669

[blink1073]

Kicking to pick up windows skips

[blink1073]

CI failures are unrelated and captured in #677. Thanks again @vkaidalov!

[welcome]

Congrats on your first merged pull request in this project! 🎉

Thank you for contributing, we are very proud of you! ❤️