rustpkg: Set exit codes properly and make tests take advantage of that

r? @metajack When I started writing the rustpkg tests, task failure didn't set the
exit code properly. But bblum's work from July fixed that. Hooray! I
just didn't know about it till now.

So, now rustpkg uses exit codes in a more conventional way, and some of
the tests are simpler.

The bigger issue will be to make task failure propagate the error message.
Right now, rustpkg does most of the work in separate tasks, which means if
a task fails, rustpkg can't distinguish between different types of failure
(see #3408)