Support keyword arguments when checking for valid arity.

This changes the error message that was included in the `ArgumentError`
raised when arity was incorrect. It used to match the default Ruby
message, it now uses our enhanced error message. While it would be
technically more correct to emulate the Ruby one, ours is more useful and the
logic required would now have to be more complicated to support keyword
arguments.

Fixes #431.

R @alindeman @myronmarston