Add error classes for drag-drop validation

- InvalidPositionError for out-of-bounds positions
- PositionOccupiedError for occupied slot conflicts

app/errors/api/v1/invalid_position_error.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+module Api
+  module V1
+    class InvalidPositionError < GranblueError
+      def code
+        'invalid_position'
+      end
+
+      def message
+        @data || 'Invalid position specified'
+      end
+    end
+  end
+end

app/errors/api/v1/position_occupied_error.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+module Api
+  module V1
+    class PositionOccupiedError < GranblueError
+      def code
+        'position_occupied'
+      end
+
+      def message
+        @data || 'Position is already occupied'
+      end
+    end
+  end
+end