Abandon fetching offline presence on 400 error

It seems like some servers return a M_UNRECOGNIZED code instead of
M_FORBIDDEN or "offline" when trying to fetch the presence for a user
and it's not available.

Fixes https://github.com/mirukana/mirage/issues/116

src/backend/errors.py

@@ -33,6 +33,12 @@ class MatrixError(Exception):
         return cls(response.transport_response.status, response.status_code)
 
 
+@dataclass
+class MatrixUnrecognized(MatrixError):
+    http_code: int = 400
+    m_code:    str = "M_UNRECOGNIZED"
+
+
 @dataclass
 class MatrixInvalidAccessToken(MatrixError):
     http_code: int = 401