If, for whatever reason, a FSMO role has no fSMORoleOwner attribute, Samba-tool shows an 'uncaught exception'

i.e. 

samba-tool fsmo seize --role=all

ERROR(<type 'exceptions.KeyError'>): uncaught exception - 'No such element'
File "/usr/lib/python2.7/dist-packages/samba/netcmd/__init__.py", line 175, in _run
  return self.run(*args, **kwargs)
File "/usr/lib/python2.7/dist-packages/samba/netcmd/fsmo.py", line 345, in run
  versionopts, force)
File "/usr/lib/python2.7/dist-packages/samba/netcmd/fsmo.py", line 301, in seize_dns_role
  master_owner = get_fsmo_roleowner(samdb, m.dn)
File "/usr/lib/python2.7/dist-packages/samba/netcmd/fsmo.py", line 42, in get_fsmo_roleowner
  master_owner = res[0]["fSMORoleOwner"][0]

This is caused by this code in fsmo.py:

def get_fsmo_roleowner(samdb, roledn):
    """Gets the owner of an FSMO role

    :param roledn: The DN of the FSMO role
    """
    res = samdb.search(roledn,
                       scope=ldb.SCOPE_BASE, attrs=["fSMORoleOwner"])
    assert len(res) == 1
    master_owner = res[0]["fSMORoleOwner"][0]
    return master_owner

As 'fSMORoleOwner' is a single-valued attribute, it can only exist or not, there cannot be more than one instance, so the last part of the above code should be:

    if len(res) == 0:
        raise Exception('"%s" does not have a FSMO roleowner' % roledn)
    master_owner = res[0]["fSMORoleOwner"][0]
    return master_owner

This will print a message and exit if there is no 'fSMORoleOwner' attribute found.

Patch to follow.