Include exec plugin stderr with wrapped error

[dhaiducek]

This allows external plugins to provide more error details aside from just exit status 1 inside the error. This also aligns with how other internal generators do error handling, for instance:

kustomize/plugin/builtin/helmchartinflationgenerator/HelmChartInflationGenerator.go

Lines 177 to 184 in b154361

	if err != nil {
	helm := p.h.GeneralConfig().HelmConfig.Command
	err = errors.WrapPrefixf(
	fmt.Errorf(
	"unable to run: '%s %s' with env=%s (is '%s' installed?): %w",
	helm, strings.Join(args, " "), env, helm, err),
	stderr.String(),
	)

To illustrate the difference, this error:

failure in plugin configured via /var/folders/mt/6766yzb50jlb9t0x6gvl0nz00000gn/T/kust-plugin-config-280379728; exit status 1

Would be populated with relevant details:

Error: error processing the PolicyGenerator file '/var/folders/mt/6766yzb50jlb9t0x6gvl0nz00000gn/T/kust-plugin-config-280379728': the PolicyGenerator configuration file is invalid: yaml: unmarshal errors:
  line 10: field consolidatManifests found but not defined in type policyDefaults - did you mean 'consolidateManifests'?
: failure in plugin configured via /var/folders/mt/6766yzb50jlb9t0x6gvl0nz00000gn/T/kust-plugin-config-280379728; exit status 1

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[k8s-ci-robot]

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[dhaiducek]

@koba1t @varshaprasad96 I've fixed the linting issue, and I suspect the MacOS failure previously was a flake. Would you please re-approve the GitHub Action run for the latest commit?

[dhaiducek]

CI is passing! 🎉

[varshaprasad96]

Thanks for the PR @dhaiducek! Looks good to me.
/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dhaiducek, varshaprasad96

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