cleanup(generator): use `Status` factory functions

[alevenberg]

https://github.com/googleapis/google-cloud-cpp/issues/14107

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This change is 

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 80.00000% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 93.23%. Comparing base (bd6390c) to head (b0c5d32). Report is 5 commits behind head on main.

Files	Patch %	Lines
generator/internal/service_code_generator.cc	0.00%	2 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14220      +/-   ##
==========================================
- Coverage   93.24%   93.23%   -0.01%     
==========================================
  Files        2206     2206              
  Lines      192142   192146       +4     
==========================================
- Hits       179156   179154       -2     
- Misses      12986    12992       +6     

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