bug#22380: 24.5; erc doesn't handle the ! prefix

[David Edmondson]

Connect to a new IRC server at $employer (over SSL using built-in
gnutls, but that doesn't seem relevant).

See the errors (copied from the *Messages* buffer):

error in process filter: erc-channel-receive-names: Unknown prefix char `33'
error in process filter: Unknown prefix char `33'

Examining the code, `erc-channel-receive-names' is trying to map prefix
characters on the name to particular user capabilities. The set of
allowed prefixes can be read from the server by `erc-parse-prefix'. In
this case, the server is returning:
        "(yqaohvV)!~&@%+-"

One of the users in a joined channel has the !/y prefix.

`erc-channel-receive-names' doesn't know about the !/y prefix, so it
emits an error at line 4793:
                        (_ (error "Unknown prefix char `%S'" ch) voice))

It would be preferable if this was simply a notice rather than an error
(or even silent). Adding support for the !/y prefix would also be great,
but this problem would just re-arise if another currently unknown prefix
is added by a server.

Thanks for emacs!

In GNU Emacs 24.5.1 (x86_64-apple-darwin15.0.0, NS apple-appkit-1404.11)
 of 2015-10-05 on heart-of-gold
Windowing system distributor `Apple', version 10.3.1404
Configured using:
 `configure --prefix=/usr/local/Cellar/emacs/24.5
 --enable-locallisppath=/usr/local/share/emacs/site-lisp
 --infodir=/usr/local/Cellar/emacs/24.5/share/info/emacs --with-xml2
 --without-dbus --with-gnutls --with-imagemagick --with-ns
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Important settings:
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Major mode: Emacs-Lisp

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  erc-fill-mode: t
  erc-pcomplete-mode: t
  erc-autojoin-mode: t
  erc-stamp-mode: t
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  erc-noncommands-mode: t
  erc-keep-place-mode: t
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  auto-composition-mode: t
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Recent messages:
Auto-saving...done
Window Switching Mode On...
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Window Switching Mode On...
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Window Switching Mode Off.
Undo!

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