Bug #1546 for Class-Accessor: Allow easier overriding of exception throwing in generated accessors

In trying to integrate Class::Accessor with an in-house exception hierarchy that uses Exception::Class, I've run into an annoying artifact. I'd rather throw my own exceptions than simply use Carp::Croak, but to do this requires me to override make_ro_accessor and make_wo_accessor, then keep up with whatever changes you make in those routines. The attached patch creates a new function make_exception which is used by mk_accessors to create the actual exception throwing code. That way one can just override make_exception instead of all the other generation code. I've included POD and a separate test file for it called Accessor-Exception.t (this avoids having to create two separate packages inside of Accessor.t. I'd appreciate any comments if you don't like it. Thanks. -- j.