Propagate error response codes to clients

[Forkk]

The server should at least let the client know about any error response code sent by the IRC server. Perhaps the client will show these with a status() message at the bottom of the screen, rather than just logging them away in the network buffer like Quassel does (though perhaps we should log them in the network buffer too). This has the advantage of making them more visible, but we should also keep in mind that status messages only show for 5 seconds, so the user could easily miss them.

One potential solution would be to have status messages clear when the user presses Ctrl+L or something, rather than simply waiting 5 seconds. This allows us to make sure the user sees them.

This issue also raises the question of dealing with unknown response codes. Do we show them to the user as status messages or not? I'm not sure what the answer to this is.