Fail early if cargo build returns non-zero exit code

[KodrAus]

Looks like something changed with the --build-plan command so it's no longer linking up dependencies. This PR just inherits the stderr so we can see what cargo is actually doing and blows up if the exit code is non-zero rather than trying to parse an empty build plan.

[KodrAus]

I logged rust-lang/cargo#6082 to track an error we've started hitting when attempting to get a build plan in recent nightlys.

[LukasKalbertodt]

Good idea! I already guessed using the build-plan feature would bring some problems with it down the road °_°