Code review comment for lp:~javier.collado/utah/bug1087620

I've replaced calls to `subprocess.Popen` with calls to
`subprocess.check_output` so that `bsdtar` return code is checked properly. The
`subprocess.CalledProcessError` exception will be caught an re-raised with a
friendlier error message.

Aside from that, I've also refactored `ISO.dump` to return just the file
contents (or raise an exception if the file cannot be extracted).