Handle non-zero exit codes from tempest script

Normally, catching a subprocess.CalledProcessError would suffice
since that exception is thrown when the subprocess exits with a
non-zero code. However this exception is only thrown when using
specific methods like check_output which we no longer use.

This commit now has us explicitly checking the process return code.

Change-Id: I799f92836fba41e718570e2f7affb27f11c26156
Merge "Handle non-zero exit codes from tempest script"