remove CellInvalidWallError, more refactoring

cell.py

@@ -99,12 +99,10 @@ class Cell:
         """
         return self._centre
 
-    def wall_exists(self, wall: int) -> bool:
+    def wall_exists(self, wall: CellWallLabel) -> bool:
         """
         returns True if a given cell wall exists, or false otherwise.
         """
-        if wall not in self._walls:
-            raise errors.CellInvalidWallError(wall)
         return self._walls[wall].exists
 
     # def break_walls_r(self, i: int, j: int) -> None:
@@ -121,8 +119,8 @@ class Cell:
         if not self._window:
             return
 
-        for _, wall in self._walls.items():
+        for label in CellWallLabel:
-            wall.draw()
+            self._walls[label].draw()
 
     def draw_move(self, to_cell: 'Cell', undo: bool = False) -> None:
         """

errors.py

@@ -1,17 +1,3 @@
-class CellInvalidWallError(Exception):
-    """
-    CellInvalidWallError is raised when the program tries to specify a Cell's
-    Wall that does not exist.
-    """
-
-    def __init__(self, wall: int, *args):
-        super().__init__(args)
-        self.wall = wall
-
-    def __str__(self):
-        return f"Invalid Cell Wall (wall int: {self.wall}) specified."
-
-
 class CellInvalidError(Exception):
     """
     CellInvalidError is raised when the program tries to create a Cell whose