samba-tool: fsmo.py throws an uncaught exception if no

samba-tool: fsmo.py throws an uncaught exception if no

fSMORoleOwner attribute

This will fix bug 11613 where a user got the uncaught exception when trying
to seize an FSMO role that didn't have the required attribute.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11613

Signed-off-by: Rowland Penny <repenny241155@gmail.com>
Reviewed-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Uri Simchoni <uri@samba.org>
Autobuild-User(master): Uri Simchoni <uri@samba.org>
Autobuild-Date(master): Sun Dec  6 00:33:10 CET 2015 on sn-devel-104