fix: map 504 errors to DEADLINE_EXCEEDED

[tritone]

This is needed to test behavior with DEADLINE_EXCEEDED errors for gRPC. GCS is still returning these errors in some cases for internal timeouts so we need to be able to test this.

Fixes #<issue_number_goes_here>

It's a good idea to open an issue first for discussion.

• Tests pass
• Appropriate changes to README are included in PR

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.68%. Comparing base (61bce3c) to head (5e40848).
Report is 29 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #663      +/-   ##
==========================================
- Coverage   98.69%   98.68%   -0.01%     
==========================================
  Files          50       50              
  Lines        8492     8461      -31     
==========================================
- Hits         8381     8350      -31     
  Misses        111      111              

Flag	Coverage Δ
unittests	98.68% <ø> (-0.01%)	⬇️

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