Rover: calc_steering_to_heading updates yaw error

this allow the vehicle's speed to be reduced based on the heading error

APMrover2/mode.cpp

@@ -430,7 +430,10 @@ void Mode::calc_steering_from_lateral_acceleration(float lat_accel, bool reverse
 // calculate steering output to drive towards desired heading
 void Mode::calc_steering_to_heading(float desired_heading_cd, float rate_max, bool reversed)
 {
-    // calculate yaw error (in radians) and pass to steering angle controller
+    // calculate yaw error so it can be used for reporting and slowing the vehicle
+    _yaw_error_cd = wrap_180_cd(desired_heading_cd - ahrs.yaw_sensor);
+
+    // call heading controller
     const float steering_out = attitude_control.get_steering_out_heading(radians(desired_heading_cd*0.01f),
                                                                          rate_max,
                                                                          g2.motors.limit.steer_left,