Enhance validation code for triggers

[savitaashture]

Changes

The PR make use of knative.dev/pkg apis errs pacakge to report better error message and multiple validation error at once which intern helps to user to know the error at once.

Signed-off-by: Savita Ashture sashture@redhat.com

Submitter Checklist

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• Includes tests (if functionality changed/added)
• Includes docs (if user facing)
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Release Notes

Triggers validation now reports all the error at once with more detailed information.

[tekton-robot]

The following is the coverage report on the affected files.
Say /test pull-tekton-triggers-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/apis/triggers/v1alpha1/trigger_validation.go	91.2%	93.8%	2.6

[dibyom]

Nice!
/approve
/lgtm

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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