cleanup(spanner): 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 84.61538% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 93.23%. Comparing base (2808049) to head (021d270). Report is 19 commits behind head on main.

Files	Patch %	Lines
...loud/spanner/internal/partial_result_set_source.cc	0.00%	2 Missing :warning:
...ogle/cloud/spanner/testing/pick_random_instance.cc	0.00%	2 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14210      +/-   ##
==========================================
- Coverage   93.78%   93.23%   -0.56%     
==========================================
  Files        2293     2206      -87     
  Lines      202921   192155   -10766     
==========================================
- Hits       190313   179158   -11155     
- Misses      12608    12997     +389     

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[alevenberg]

PR is ready for review