[WFCORE-37] Don't throw an exception if a handler is added via the ad…

[jamezp]

…d-handler operation. Also fix the model validation to keep the handler names unique when associated with a logger or async-handler.

https://issues.jboss.org/browse/WFCORE-37

If adding a handler and then adding it to a logger or async-handler in a composite operation via the add-handler operation a failure would occur because the add operation already adds the handler as it's found in the model already. This just ignores the error and logs a trace message instead. The add-handler operation shouldn't fail if the handler is already assigned.