Infrom user why rt policy could not be set, infrom if is set. (backport

#1705) (#1709)

controller_manager/src/ros2_control_node.cpp

@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+#include <errno.h>
 #include <algorithm>
 #include <chrono>
 #include <memory>
@@ -52,12 +53,19 @@ int main(int argc, char ** argv)
       {
         if (!realtime_tools::configure_sched_fifo(kSchedPriority))
         {
-          RCLCPP_WARN(cm->get_logger(), "Could not enable FIFO RT scheduling policy");
+          RCLCPP_WARN(
+            cm->get_logger(),
+            "Could not enable FIFO RT scheduling policy: with error number <%i>(%s). See "
+            "[https://control.ros.org/master/doc/ros2_control/controller_manager/doc/userdoc.html] "
+            "for details on how to enable realtime scheduling.",
+            errno, strerror(errno));
         }
       }
       else
       {
-        RCLCPP_INFO(cm->get_logger(), "RT kernel is recommended for better performance");
+        RCLCPP_INFO(
+          cm->get_logger(), "Successful set up FIFO RT scheduling policy with priority %i.",
+          kSchedPriority);
       }
 
       // for calculating sleep time