Fix traceback when sar module is force-enabled

Forward port of the following sos-2.2 fix:

commit d4e3bcc3ed247d8d9a1228c3661b6456f534daf4
Author: Bryn M. Reeves <bmr@redhat.com>
Date:   Mon Aug 12 18:42:48 2013 +0100

    Fix traceback when sar module is force-enabled

    The sar plug-in assumes that when it runs the /var/lib/sa
    directory and binaries exist (they are part of checkenabled() for
    this module).

    This leads to a traceback when the plug-in lists a non-existent
    directory on hosts where sar is not installed but '-o sar' is
    specified on the command line.

    Catch the exception and exit gracefully.

    (Edited by bmr to improve error message logged).

    Signed-off-by: David Kutálek <dkutalek@redhat.com>
    Signed-off-by: Bryn M. Reeves <bmr@redhat.com>