Append target related error message to corresponding buffer

[SilverRainZ]

As shown in https://www.alien.net.au/irc/irc2numerics.html, IRC server may returns various error message. Currently Srain always append the error message to current buffer:

https://github.com/SilverRainZ/srain/blob/52351895ffabc38c2ac6839f1dd6aad08897f55a/src/core/app_irc_event.c#L1750

In fact, some of these messages have related to specified buffer. For example, ERR_NOCHANMODES(477) is returned when client try to join a invite only channel. Srain should add this messgae to buffer of this channel but no to current buffer.

• 477 <channel> Cannot join channel (+r) - you need to be identified with services - see https://freenode.net/kb/answer/registration
• 473 <channel> Cannot join channel (+i) - you must be invited
• 475