Catch 'Internal Server Error' on update

When there is an internal server error, we won't log an error
and hold off updates for a while. This is an error on the
provider's side and the client must retry in order to update
the hostname.

This error was seen with selfhost.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>
Signed-off-by: Stefan Schantl <stefan.schantl@ipfire.org>