Ugly reporting of failing validators on Create Issue

When a (custom) validator is placed on the "Create Issue" transition, and the validator fails, the error message:

• Is placed at the top of the screen, rather than associating with any field.
• Contains the name of the exception class (com.atlassian.jira.workflow.WorkflowException, for instance), which is confusing and irrelevant to end users.

We should clean up this error handling, particularly involving WorkflowExceptions, which validators throw.