fix: print concrete error from fillPlugin in logs

[randmonkey]

What this PR does / why we need it:

Print the concrete error returned from fillPlugin function in logs.

Which issue this PR fixes:

part of #5341
Special notes for your reviewer:

REVIEW: how to write changelog for this?

PR Readiness Checklist:

Complete these before marking the PR as ready to review:

• the CHANGELOG.md release notes have been updated to reflect any significant (and particularly user-facing) changes introduced by this PR

[codecov]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (b929f67) 68.0% compared to head (925b7e0) 68.0%.
Report is 2 commits behind head on release/2.12.x.

Files	Patch %	Lines
internal/manager/run.go	44.4%	4 Missing and 1 partial ⚠️
internal/dataplane/deckgen/generate.go	66.6%	1 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff                @@
##           release/2.12.x   #5346     +/-   ##
================================================
- Coverage            68.0%   68.0%   -0.1%     
================================================
  Files                 165     165             
  Lines               19344   19352      +8     
================================================
+ Hits                13159   13164      +5     
- Misses               5397    5401      +4     
+ Partials              788     787      -1     

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[tao12345666333]

#5347 will fix the CI failure

[czeslavo]

Nits: we could use WithError to log the error as a log field.