Most exploits don't check nil for generate_payload_exe, they just
assume they will always have a payload. If the method returns nil,
it ends up making debugging more difficult. Instead of checking nil
one by one, we just raise.
See the complaint on #4039. This doesn't fix that particular
issue (it's somewhat unrelated), but does solve around
a file parsing problem reported by @void-in
Christian Mehlmauer
Brent Cook
James Lee
wchen-r7
HD Moore
URI Assassin
jvazquez-r7