Replace "This is fatal error" in several exception messages with "This is a fatal error"
Various fixes for overloading operator conflict detection

If a role consumes another role, the consuming role's overloaded operators
just win, rather than being a conflict. Fallback setting conflicts are still a
conflict, however.

If there are operator conflicts during role summation we check the method
bodies and only make it a conflict if the bodies are different. This handles
the case where two roles got their overloading from the same source (another
role).
Merge pull request #79 from moose/topic/smarter-overloading-conflict-detection

Fix overloading conflict detection