Code review comment for lp:~lool/linaro-image-tools/lp-711573

As I said in the bug, this won't be enough to fix the bug; lmc will still fail because we're passing cmd_runner.Popen() an argument it is not expecting.

Also, we should not wrap both umount statements in a single try/except block as if the first one fails the second won't execute. This is not the same behavior exhibited by the previous code, which was using raw Popen() to ignore failures in the subprocesses