CNC workspace planes for G5

According to LinuxCNC, G5 should throw an error if the current plane mode isn't XY.

Marlin/Marlin_main.cpp

   inline void gcode_G5() {
     if (IsRunning()) {
 
+      #if ENABLED(CNC_WORKSPACE_PLANES)
+        if (workspace_plane != PLANE_XY) {
+          SERIAL_ERROR_START();
+          SERIAL_ERRORLNPGM(MSG_ERR_BAD_PLANE_MODE);
+          return;
+        }
+      #endif
+
       gcode_get_destination();
 
       const float offset[] = {
     millis_t next_idle_ms = millis() + 200UL;
 
     #if N_ARC_CORRECTION > 1
-      int8_t count = N_ARC_CORRECTION;
+      int8_t arc_recalc_count = N_ARC_CORRECTION;
     #endif
 
     for (uint16_t i = 1; i < segments; i++) { // Iterate (segments-1) times
       }
 
       #if N_ARC_CORRECTION > 1
-        if (--count) {
+        if (--arc_recalc_count) {
           // Apply vector rotation matrix to previous r_P / 1
           const float r_new_Y = r_P * sin_T + r_Q * cos_T;
           r_P = r_P * cos_T - r_Q * sin_T;
       #endif
       {
         #if N_ARC_CORRECTION > 1
-          count = N_ARC_CORRECTION;
+          arc_recalc_count = N_ARC_CORRECTION;
         #endif
 
         // Arc correction to radius vector. Computed only every N_ARC_CORRECTION increments.
     // motion control system might still be processing the action and the real tool position
     // in any intermediate location.
     set_current_to_destination();
-  }
-#endif
+  } // plan_arc
+
+#endif // ARC_SUPPORT
 
 #if ENABLED(BEZIER_CURVE_SUPPORT)
 

Marlin/language.h

 #define MSG_ERR_MATERIAL_INDEX              "M145 S<index> out of range (0-1)"
 #define MSG_ERR_M355_NONE                   "No case light"
 #define MSG_ERR_M421_PARAMETERS             "M421 incorrect parameter usage"
+#define MSG_ERR_BAD_PLANE_MODE              "G5 requires XY plane mode"
 #define MSG_ERR_MESH_XY                     "Mesh point cannot be resolved"
 #define MSG_ERR_ARC_ARGS                    "G2/G3 bad parameters"
 #define MSG_ERR_PROTECTED_PIN               "Protected Pin"