sanlock isconfigured raises an exception instead of returning result

isconfigured calls expect boolean as return value. Exception should be
raised only on critical failures such as sanlock service isn't installed,
or sanlock group does not exist.

Change-Id: Ic174c702f7f5b491975829e227d8311aed26ec90
Signed-off-by: Yaniv Bronhaim <ybronhei@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/22184
Reviewed-by: Alon Bar-Lev <alonbl@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>