feat: AppSet PR generator return 0 results upon repo not found instead of failing

[reggie-k]

Closes #22550

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 60.25%. Comparing base (0479433) to head (016a805).
⚠️ Report is 800 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #23447      +/-   ##
==========================================
+ Coverage   60.21%   60.25%   +0.04%     
==========================================
  Files         345      346       +1     
  Lines       59173    59212      +39     
==========================================
+ Hits        35629    35677      +48     
+ Misses      20660    20654       -6     
+ Partials     2884     2881       -3     

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[reggie-k]

Thanks everyone for the review, the PR is still draft and I outlined some concerns and possible implementations on the issue.
The final version will analyze the http status code where applicable, and analyze the error only for providers that don't provide a standard http response code.

[reggie-k]

The PR is now ready for review.
I took the approach of handling the HTTP status code for providers that return it in a standard way, and then string-analyzed the error only for those that don't return an http status.
In addition to unit tests, also tested manually for all providers except for BitBucket Server (more complex setup required, but will update in case I manage to manually test it as well).

[pasha-codefresh]

LGTM, only small comment from my side

[nitishfy]

Left a small comment, otherwise LGTM!