feat: auth error handling

backend/constants.py

@@ -6,7 +6,11 @@ class MESSAGES(str, Enum):
 
 
 class ERROR_MESSAGES(str, Enum):
+    def __str__(self) -> str:
+        return super().__str__()
+
     DEFAULT = lambda err="": f"Something went wrong :/\n{err if err else ''}"
+    ENV_VAR_NOT_FOUND = "Essential environment variable not found. Terminating now."
     INVALID_TOKEN = (
         "Your session has expired or the token is invalid. Please sign in again."
     )