Add check for planning scene monitor connection, with 5 sec delay

**Note:** *this is a fixed PR from https://github.com/ros-planning/moveit_ros/pull/446*

Fix for error that sometimes occurs when the Rviz Motion Planning plugin is launch with the move_group node in the same launch file. Before making a service call to get an updated planning scene, this causes it to wait for the service to exist up to 5 seconds before showing the error

```
Failed to call service /get_planning_scene
```

The 5 seconds is arbitrary but is enough to allow the timing issue to usually work out. This fix doesn't change any functionality, just helps remove the error message.

This PR also helps clarify some of the related error messages and changes one to just a warning since the Rviz motion planning plugin will still continue to work and there are use cases when its ok to not have move_group launched.