[feat] add basic domain

domain/error.py

@@ -0,0 +1,18 @@
+class DomainError(Exception):
+    pass
+
+
+class UserError(DomainError):
+    pass
+
+
+class UserNotFoundError(UserError):
+    def __init__(self, user_id: str) -> None:
+        super().__init__('user_not_found')
+        self.user_id = user_id
+
+
+class UserConflictError(UserError):
+    def __init__(self, email: str) -> None:
+        super().__init__('user_conflict')
+        self.email = email