Fix entry point lookups without any custom entry points.
The recent fix for looking up third party entry points was failing if

there weren't any custom entry points registered. Typing a command that

RBTools doesn't supply would raise a StopIteration error. This happened

because the lookup assumed the existence of at least one entry.
It's now a bit more bullet-proof and catches that StopIteration

cleanly.

Typed an invalid RBTools command name and saw it give me a proper error

instead of StopIteration.