StatelessChecker: add helpful information to find stateful behavior (patch included)

When the stateless check fails, one has to search for the offending (stateful) behavior manually, which can be tedious in complex components (containing multiple behaviors, maybe even spread across classes through inheritance)

Attached patch adds to the exception message the class name(s) of stateful Behaviors(s), if any.

Note: I did not test the code, since my project is not running on the wicket sources. Should be working fine, though.