Fix exit of subprocess in case it was terminated by signal

If child process of rootwrap is terminated by signal, exit code should
be 128 + N, where N is number of signal. According [1]
subprocess.Popen.returncode returns -N. sys.exit interprets negative
signed byte as unsigned thus original exit code is 256 - N instead of
128 + N.

[1]
https://docs.python.org/2/library/subprocess.html#subprocess.Popen.returncode

Change-Id: I43b4c959a98f8f620466c77de4cd5b724b09e5a8
Closes-Bug: #1364822
Merge "Fix exit of subprocess in case it was terminated by signal"