Added more information to error message for ambiguous default attack

Fixes #1271.

http://jenkins.buttonweavers.com:8080/job/buttonmen-blackshadowshade/440/
My two cents is that I don't like the form-of-a-question way, because the player can't *answer* the question, i.e. there's not a yes/no box for them to check. (And there shouldn't be, that would be weird. :^)

I like the idea of just enumerating what the possible options are that make the attack ambiguous, and optionally why the player would care, i.e.

* You could perform a Power Attack (which would trigger the Doppelganger skill) or a Skill Attack (which would not).
* You could perform a Berserk Attack (which would trigger the Berserk skill) or a Skill Attack (which would not).

What do you think of that approach?

("optionally why the player would care" in that I think you could drop the parentheticals, and let the player figure out (as a learning experience) what the difference is. I think it's fine to be more helpful; experienced players may feel like it's condescending, but enh.)