Log warning when gen_protos is not found

[AnandInguva]

For internal tests, tar ball is created by copybara and it doesn't include gen_protos.py. In the PR #28385, we provide hard RuntimeError when error originates from gen_protos.py. By pass the error when gen_protos is not found.

FYI, externally built sdist contains gen_protos.py but for internal imports, we don't. Since it is a build time dependency for building proto files, it is not necessary to raise RuntimeError while installing tar ball using pip install <>.tar.gz

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[codecov]

Codecov Report

Merging #29025 (e02b03c) into master (a6917d4) will decrease coverage by 0.02%.
Report is 10 commits behind head on master.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master   #29025      +/-   ##
==========================================
- Coverage   38.40%   38.39%   -0.02%     
==========================================
  Files         686      686              
  Lines      101609   101640      +31     
==========================================
  Hits        39028    39028              
- Misses      61001    61032      +31     
  Partials     1580     1580              

Flag	Coverage Δ
python	30.00% <0.00%> (-0.02%)	⬇️

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Files	Coverage Δ
sdks/python/setup.py	0.00% <0.00%> (ø)

... and 4 files with indirect coverage changes

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[AnandInguva]

Passing cl: https://critique.corp.google.com/cl/573923057/analysis

R: @chamikaramj

[github-actions]

Stopping reviewer notifications for this pull request: review requested by someone other than the bot, ceding control

[chamikaramj]

LGTM.