Add 503 and 504 errorcodes to list

[petrekova]

Sometimes (especially in the night), the Jodel API decides to throw either 504's (e.g. when creating/refreshing an account) or 503's (when looking up comments).

These codes should be added to the error code list.

Maybe it should also be indicated, that creating new accounts should be also catched for exceptions (especially 504's).