Ensure rustpkg test fails if tests failed

It previously set the exit status, but the main wrapper paved over that
with an exit code of 0.

Closes #9761
auto merge of #10254 : sfackler/rust/rustpkg-test-failure, r=alexcrichton

It previously set the exit status, but the main wrapper paved over that
with an exit code of 0.

Closes #9761