gazebo spawn_model fails

[Carter90]

So sometime when I'm swapping out the models the spawn_model fails so need to add a check and reattempt

  File "/home/carter/ChiliHouse/src/mobility/src/mobility/swarmie.py", line 341, in _plant
    self.spawn_model("plant_"+str(msg.id), self.thirst_model, "", pose,"world")
  File "/opt/ros/kinetic/lib/python2.7/dist-packages/rospy/impl/tcpros_service.py", line 435, in __call__
    return self.call(*args, **kwds)
  File "/opt/ros/kinetic/lib/python2.7/dist-packages/rospy/impl/tcpros_service.py", line 525, in call
    raise ServiceException("transport error completing service call: %s"%(str(e)))
ServiceException: transport error completing service call: unable to receive data from sender, check sender's logs for details