blob/gcsblob: fix error checking for NotFound errors

[BrennaEpp]

Fixes #3528

I have not tested this but it is a recommended change that I believe should fix the linked issue.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.29%. Comparing base (da0b8ec) to head (fb89218).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3529      +/-   ##
==========================================
- Coverage   73.31%   73.29%   -0.02%     
==========================================
  Files         113      113              
  Lines       15190    15190              
==========================================
- Hits        11136    11134       -2     
- Misses       3275     3277       +2     
  Partials      779      779              

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