Return the same error message from /login when password is incorrect and when account doesn't exist.

changelog.d/12738.misc

@@ -0,0 +1 @@
+Change error message when attempting to login to a non-existent account to match that returned when the account exists but the password is invalid. Contributed by Daniel Aloni.

synapse/handlers/auth.py

@@ -1204,7 +1204,7 @@ async def validate_login(
                     await self._failed_login_attempts_ratelimiter.can_do_action(
                         None, (medium, address)
                     )
-                raise LoginError(403, "", errcode=Codes.FORBIDDEN)
+                raise LoginError(403, "Invalid password", errcode=Codes.FORBIDDEN)
 
             identifier_dict = {"type": "m.id.user", "user": user_id}