logging: Don't use Error when logging 4xx responses

[jku]

The main change is the log type change in handleTimestampAPIError() but there's other small tweaks:

• in the error.go case also avoid the misleading "exiting with error" message and use the original message instead.
• make sure the two handlers use the same logic to choose between error and warning

This came up in sigstore/rekor-tiles#483

[codecov]

Codecov Report

❌ Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 17.32%. Comparing base (6fd19b0) to head (a0b03e6).
⚠️ Report is 471 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/api/error.go	0.00%	5 Missing ⚠️
...kg/generated/restapi/configure_timestamp_server.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1159       +/-   ##
===========================================
- Coverage   52.85%   17.32%   -35.53%     
===========================================
  Files          20       55       +35     
  Lines        1209     3705     +2496     
===========================================
+ Hits          639      642        +3     
- Misses        509     3010     +2501     
+ Partials       61       53        -8     

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