Add ordering dependency and make lomiri-url-service more robust

Add an ordering dependency on graphical-session.target.

Make sure an exception is thrown if OverlayTrackerMir cannot connect to mir, otherwise it will later crash without explanation when it first tries to use the connection. This should help with future debugging. Make overlay_tracker_new() return NULL in case of the above. Finally, handle this in lomiri-url-dispatcher and print error messages.

This fixes #9 (closed).