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

test/mavros_posix_tests_missions.test

@@ -5,6 +5,7 @@
     <arg name="headless" default="true"/>
     <arg name="gui" default="false"/>
     <arg name="est" default="ekf2"/>
+    <arg name="respawn_gazebo" default="true"/>
 
     <include file="$(find px4)/launch/mavros_posix_sitl.launch">
         <arg name="ns" value="$(arg ns)"/>
@@ -12,6 +13,7 @@
         <arg name="gui" value="$(arg gui)"/>
         <arg name="vehicle" value="standard_vtol"/>
         <arg name="est" value="$(arg est)"/>
+        <arg name="respawn_gazebo" value="$(arg respawn_gazebo)"/>
     </include>
 
     <group ns="$(arg ns)">