reevaluate error handling for +s channels

[slingamn]

from @progval :

>>> mode #nonexistent +o second
:testnet.ergo.chat 403 second #nonexistent :No such channel
>>> mode #secret +o second
:testnet.ergo.chat 482 second #secret :You're not a channel operator

This distinction leaks the existence of the secret channel. This is not a very big deal since you can always find out whether a secret channel exists by trying to JOIN it. Technically, this is silent whereas a successful JOIN notifies the occupants; however, failure to JOIN a secret channel that is also protected by +k or +i (which would be typical of channels that are actually sensitive) would also be silent.

[slingamn]

442 (ERR_NOTONCHANNEL) is a pretty safe thing to send that covers both cases. (TOPIC currently sends 403 for a nonexistent channel, 442 for a secret or non-secret channel where you're not a member).

PRIVMSG and NOTICE will send 404 (ERR_CANNOTSENDTOCHAN).