Add the method name to the message of InvocationService to make it easier to
find out the rootcause of the exception.

Review at http://gwt-code-reviews.appspot.com/1450803

Review by: rjrjr@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@10207 8db76d5a-ed1c-0410-87a9-c151d255dfc7