Return codes from underlying build commands are not delegated

When an underlying command fails, e.g. a grade build:
https://travis-ci.org/matzew/helloworld-android-gradle/builds/169722237#L458

the return code of that command should be used and bubbled up the chain.

Currently we are not doing this, which leads to false true effect, like here:
https://travis-ci.org/matzew/helloworld-android-gradle/builds/169722237#L465