Change #5977: Use specific error message when attempting to create a circular group hierarchy (3298)

src/group_cmd.cpp

@@ -449,7 +449,7 @@ CommandCost CmdAlterGroup(TileIndex tile, DoCommandFlag flags, uint32 p1, uint32
 
 			/* Ensure request parent isn't child of group.
 			 * This is the only place that infinite loops are prevented. */
-			if (GroupIsInGroup(pg->index, g->index)) return CMD_ERROR;
+			if (GroupIsInGroup(pg->index, g->index)) return_cmd_error(STR_ERROR_GROUP_CAN_T_SET_PARENT_RECURSION);
 		}
 
 		if (flags & DC_EXEC) {

src/lang/english.txt

@@ -4473,6 +4473,7 @@ STR_ERROR_GROUP_CAN_T_CREATE                                    :{WHITE}Can't cr
 STR_ERROR_GROUP_CAN_T_DELETE                                    :{WHITE}Can't delete this group...
 STR_ERROR_GROUP_CAN_T_RENAME                                    :{WHITE}Can't rename group...
 STR_ERROR_GROUP_CAN_T_SET_PARENT                                :{WHITE}Can't set parent group...
+STR_ERROR_GROUP_CAN_T_SET_PARENT_RECURSION                      :{WHITE}... loops in the group hierarchy are not allowed
 STR_ERROR_GROUP_CAN_T_REMOVE_ALL_VEHICLES                       :{WHITE}Can't remove all vehicles from this group...
 STR_ERROR_GROUP_CAN_T_ADD_VEHICLE                               :{WHITE}Can't add the vehicle to this group...
 STR_ERROR_GROUP_CAN_T_ADD_SHARED_VEHICLE                        :{WHITE}Can't add shared vehicles to group...