fixup to make it work

slapos/grid/slapgrid.py

@@ -61,7 +61,7 @@ from slapos.grid.svcbackend import (launchSupervisord,
                                     _getSupervisordConfigurationDirectory,
                                     _getSupervisordSocketPath)
 from slapos.grid.utils import (md5digest, dropPrivileges, SlapPopen, updateFile)
-from slapos.grid.promise import PromiseLauncher
+from slapos.grid.promise import PromiseLauncher, PromiseError
 from slapos.human import human2bytes
 import slapos.slap
 from netaddr import valid_ipv4, valid_ipv6