More informative warning of validation errors

For example, "Unrecognized property: isNew" becomes "Validation error
against schema GettingStartedOnRedirect: Unrecognized property: isNew"

Maybe also warn() should prefix everything with "EventLogging", since
the line number of the warning is in jQuery proxy().

Change-Id: I9bdc4a0d508433a5ad0b5c0936e2493371bea129