ifupdownmain: missing return statement and fixing 'network down' error checking

Ticket:
Reviewed By: Roopa, Nikhil G

Signed-off-by: Julien Fortin <julien@cumulusnetworks.com>

ifupdown/ifupdownmain.py

@@ -306,7 +306,7 @@ class ifupdownMain(ifupdownBase):
         #   is a LINK_SLAVE of a bridge (in other words the bond is
         #   part of a bridge) which is not up yet
         if self._link_master_slave:
-           if 'Network is down':
+           if 'Network is down' in errorstr:
               return True
         return False
 
@@ -318,7 +318,7 @@ class ifupdownMain(ifupdownBase):
         if self.flags.STATEMANAGER_ENABLE:
            return self.statemanager.get_ifaceobjs(ifacename)
         else:
-           None
+           return None
 
     def get_ifaceobj_first(self, ifacename):
         ifaceobjs = self.get_ifaceobjs(ifacename)