Validator pattern should be able to handle dispatch errors with a specific expression

• User impact:

  Low
• Effort points:

  1
• Affects Docs:

  Yes
• Migration Impact:

  Nil
• Similar Issues:

  None

When an error occurs in the outbound phase of the Validator pattern, there is no way of letting the caller know about it.

This should be made possible, as an optional extra property named 'errorExpression'.