Improved type-hinting and overall sanity checks implemented.

errors/__init__.py

@@ -0,0 +1 @@
+from .user import UserNotFoundError

errors/user.py

@@ -0,0 +1,10 @@
+class UserNotFoundError(Exception):
+    """HoloUser could not find user with such an ID in database"""
+
+    def __init__(self, user, user_id):
+        self.user = user
+        self.user_id = user_id
+
+        super().__init__(
+            f"User of type {type(self.user)} with id {self.user_id} was not found"
+        )