When we received server errors with no matrix M_CODE, the MatrixError
class would fall back to its first subclass that had a matching HTTP
status code. This notably caused any 401 error to be interpreted as
MatrixInvalidAccessToken, and disconnect the user.
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
