🐛 Surface workflow verification errors with doc link

[spencerschrock]

Should resolve ossf/scorecard-action#1150 once a new webapp release is cut. The underlying logic always existed for POST requests, but the implementation wasn't working due to a %v instead of a %w here:

scorecard-webapp/app/server/post_results.go

Lines 233 to 236 in 9c2f66d

	// Verify scorecard workflow.
	if err := verifyScorecardWorkflow(workflowContent); err != nil {
	return fmt.Errorf("workflow could not be verified: %v", err)
	}

After testing locally, I received the following response to the issue reporter's POST data:

{
  "code": 400,
  "message": "workflow verification failed: workflow contains global env vars or defaults, see https://github.com/ossf/scorecard-action#workflow-restrictions for details."
}

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