Added more information to error message for ambiguous default attack

Fixes #1271.

http://jenkins.buttonweavers.com:8080/job/buttonmen-blackshadowshade/440/
I'd turn this around, maybe "A Power attack will trigger the Doppelganger skill, while other attack types won't".  IMO, that would make it more clear that Doppelganger is a side-effect skill, not an attack type, which may not be totally obvious to new players.