Raise and catch a specific error during validation

This changes stack creation to return an error message instead of an
exception when (some) validation fails. It doesn't convert all validation
errors to keep the patch at a reasonable size.

Change-Id: I6364bccacbc1d447a99ea61565c96fcc90bd21f9
Partial-Bug: #1276623
Merge "Raise and catch a specific error during validation"