The module approach looks interesting. An alternative is to override the method on the rpc servlet which handles this part. Then trap the error and return a string of your choice.
The method will be the one which handles the deserialisation in the RemoteServiceServlet. Possibly http://www.gwtproject.org/javadoc/latest/com/google/gwt/user/server/rpc/AbstractRemoteServiceServlet.html#onAfterRequestDeserialized(com.google.gwt.user.server.rpc.RPCRequest)
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