refactor(generator): split *ErrorCollector

[coryan]

Part of the work for #7664

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This change is 

[codecov]

Codecov Report

Base: 93.73% // Head: 93.74% // Increases project coverage by +0.01% 🎉

Coverage data is based on head (757e29c) compared to base (e773e4e).
Patch coverage: 45.45% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10675      +/-   ##
==========================================
+ Coverage   93.73%   93.74%   +0.01%     
==========================================
  Files        1688     1690       +2     
  Lines      152699   152679      -20     
==========================================
  Hits       143135   143135              
+ Misses       9564     9544      -20     

Impacted Files	Coverage Δ
generator/generator_test.cc	100.00% <ø> (+5.63%)	⬆️
generator/internal/predicate_utils_test.cc	100.00% <ø> (+1.15%)	⬆️
generator/testing/error_collectors.cc	0.00% <0.00%> (ø)
generator/internal/descriptor_utils_test.cc	99.01% <100.00%> (+1.27%)	⬆️
...erator/internal/resolve_comment_references_test.cc	100.00% <100.00%> (+13.20%)	⬆️
generator/internal/service_code_generator_test.cc	99.29% <100.00%> (+1.37%)	⬆️
generator/testing/error_collectors.h	100.00% <100.00%> (ø)
google/cloud/pubsub/subscriber_connection_test.cc	100.00% <0.00%> (+1.21%)	⬆️
...e/cloud/pubsublite/internal/alarm_registry_impl.cc	100.00% <0.00%> (+2.94%)	⬆️

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