Better arity error messages

This adds better error messages to the arity exceptions that are
raised when stubbing a method or calling it with the wrong arity.
This should help with identifying the issue and what needs to be
addressed. The message now clearly explains the method that was
called/stubbed with the wrong arity, how many args it expected
and how many it got.
Merge pull request #206 from redding/jcr-better-arity-errors

Better arity error messages