AC_PosControl: add get_horizontal_error

libraries/AC_AttitudeControl/AC_PosControl.cpp

@@ -744,6 +744,11 @@ void AC_PosControl::update_vel_controller_xyz(float ekfNavVelGainScaler)
     update_z_controller();
 }
 
+float AC_PosControl::get_horizontal_error() const
+{
+    return norm(_pos_error.x, _pos_error.y);
+}
+
 ///
 /// private methods
 ///

libraries/AC_AttitudeControl/AC_PosControl.h

@@ -148,6 +148,9 @@ public:
     /// get_alt_error - returns altitude error in cm
     float get_alt_error() const;
     
+    // returns horizontal error in cm
+    float get_horizontal_error() const;
+
     /// set_target_to_stopping_point_z - sets altitude target to reasonable stopping altitude in cm above home
     void set_target_to_stopping_point_z();