improve error message in the agent plugin

[pingsutw]

Tracking issue

NA

Why are the changes needed?

Improve the error message

What changes were proposed in this pull request?

Update error message

How was this patch tested?

Setup process

Screenshots

Check all the applicable boxes

• I updated the documentation accordingly.
• All new and existing tests passed.
• All commits are signed-off.

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 8 lines in your changes missing coverage. Please review.

Project coverage is 36.98%. Comparing base (e21a6ad) to head (cabe46a).
Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
...yteplugins/go/tasks/plugins/webapi/agent/plugin.go	33.33%	8 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6114   +/-   ##
=======================================
  Coverage   36.98%   36.98%           
=======================================
  Files        1318     1318           
  Lines      132449   132450    +1     
=======================================
+ Hits        48984    48987    +3     
+ Misses      79212    79209    -3     
- Partials     4253     4254    +1     

Flag	Coverage Δ
unittests-datacatalog	51.58% <ø> (ø)
unittests-flyteadmin	54.06% <ø> (+0.02%)	⬆️
unittests-flytecopilot	30.99% <ø> (ø)
unittests-flytectl	62.29% <ø> (ø)
unittests-flyteidl	7.23% <ø> (ø)
unittests-flyteplugins	53.85% <33.33%> (-0.01%)	⬇️
unittests-flytepropeller	42.60% <ø> (ø)
unittests-flytestdlib	55.13% <ø> (-0.06%)	⬇️

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[eapolinario]

The unit tests failure are real: https://github.com/flyteorg/flyte/actions/runs/12381508644/job/34560236391?pr=6114. Can you take a look?