Return info from run_command on exception.

    As a follow-up from the discussion here:
    http://lists.ovirt.org/pipermail/kimchi-devel/2014-April/004600.html
    run_command needs to be updated to return some indication
    to the caller that a command did not complete. Currently,
    if there is an exception (e.g. the command does not exist),
    None is returned for the output, the error output, and
    the return code.

    msg is already created, but only printed to the log/console,
    so return that as the error message. Return -1 for the rc, which
    could be, depending on the command passed, an ignorable error
    code. Having some indication of a problem is better than None,
    though.

Signed-off-by: Christy Perez <christy@linux.vnet.ibm.com>