Added more information to error message for ambiguous default attack

Fixes #1271.

http://jenkins.buttonweavers.com:8080/job/buttonmen-blackshadowshade/440/
Thinking about this more: the "It is not clear" wording is a bit awkward.  A couple of alternate suggestions:
* phrase it as a question: "Default attack is ambiguous.  Do you want to perform a Berserk attack or not?"
* explain the problem more directly: "Default attack is ambiguous.  You may choose to perform a Berserk attack."