add enif_is_exception

[Steve Vinoski]

This patch adds a new NIF API function enif_is_exception to allow
applications to check whether an ERL_NIF_TERM is an exception or not.
It adds a new test for the new function and also documents the new
function. It also clarifies what applications are permitted to do with
the ERL_NIF_TERM returned from enif_make_badarg.

git fetch git://github.com/vinoski/otp.git enif_is_exception

Thanks to Sverker Eriksson for guidance in putting this patch together.

--steve