Correctly handle account with invalid access token

Show a popup saying the session was signed out and cleanup
the models data, instead of spamming the users with
never-ending errors.

src/backend/errors.py

@@ -33,6 +33,12 @@ class MatrixError(Exception):
         return cls(response.transport_response.status, response.status_code)
 
 
+@dataclass
+class MatrixInvalidAccessToken(MatrixError):
+    http_code: int = 401
+    m_code:    str = "M_UNKNOWN_TOKEN"
+
+
 @dataclass
 class MatrixUnauthorized(MatrixError):
     http_code: int = 401