XWork should report that Action does not Implement Action interface

If an action does not implement Action a XWorkException is thrown:

com.opensymphony.xwork.XworkException: Unable to instantiate Action, de.projectparcel.ebay.EOAImport, defined for 'eoaimport' in namespace '' – null

First the message text "null" is not nice, it comes from ClassCastException not providing a message and second, it's not clear that the error is caused by EOAImport not implementing Action.

Will attach a patch.