Fix error handling in request log template execution

[linkvt]

Fixes #15981

Proposed Changes

• Fix error handling in request log template execution
  • Add missing return statement after template execution error to prevent writing partial buffer contents after the error message has been logged.

Release Note

Fix request log output corruption when using invalid log templates

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.01%. Comparing base (c46db77) to head (728d64a).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16242      +/-   ##
==========================================
- Coverage   80.02%   80.01%   -0.01%     
==========================================
  Files         214      214              
  Lines       13317    13318       +1     
==========================================
  Hits        10657    10657              
- Misses       2298     2300       +2     
+ Partials      362      361       -1     

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[dprotaso]

/lgtm
/approve

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dprotaso, linkvt

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