CI: allow Gazebo to restart on crash (#8817)

* add respawn_gazebo arg to be used with empty_world.launch
* catch rospy sleep method's exceptions
* fix copy-paste mistake in land state failure message

launch/posix_sitl.launch

@@ -20,6 +20,7 @@
     <arg name="debug" default="false"/>
     <arg name="verbose" default="false"/>
     <arg name="paused" default="false"/>
+    <arg name="respawn_gazebo" default="false"/>
 
     <node name="sitl" pkg="px4" type="px4" output="screen"
         args="$(find px4) $(arg rcS)">
@@ -32,6 +33,7 @@
         <arg name="debug" value="$(arg debug)" />
         <arg name="verbose" value="$(arg verbose)" />
         <arg name="paused" value="$(arg paused)" />
+        <arg name="respawn_gazebo" value="$(arg respawn_gazebo)"/>
     </include>
 
     <node name="$(anon vehicle_spawn)" output="screen" pkg="gazebo_ros" type="spawn_model"