Tools: abort SITL start if gzserver fails

Sometimes gzserver seems to not start or not start fast enough. In this
case, instead of stalling forever, it would be nice to abort.

Tools/sitl_run.sh

@@ -105,8 +105,11 @@ elif [ "$program" == "gazebo" ] && [ ! -n "$no_sim" ]; then
 			fi
 		fi
 		SIM_PID=$!
-		gz model --spawn-file="${src_path}/Tools/sitl_gazebo/models/${model}/${model}.sdf" --model-name=${model} -x 1.01 -y 0.98 -z 0.83
 
+		if gz model --spawn-file="${src_path}/Tools/sitl_gazebo/models/${model}/${model}.sdf" --model-name=${model} -x 1.01 -y 0.98 -z 0.83 2>&1 | grep "An instance of Gazebo is not running."; then
+			echo "Start of gzserver failed"
+			exit 1
+        fi
 
 		if [[ -n "$HEADLESS" ]]; then
 			echo "not running gazebo gui"