Add MOTD command to ERC

Add MOTD command to ERC

* lisp/erc/erc-backend.el (erc-server-402, erc-server-402-functions):
Add new response handler and hook.
* lisp/erc/erc.el (erc-cmd-MOTD): New function to shield erc-network
from handling post-connection MOTD replies.  Thanks to Emanuel Berg
for reporting this (bug#62151).
(erc-message-english-s402): Define new ERR_NOSUCHSERVER message
template.
* test/lisp/erc/erc-scenarios-base-misc-regressions.el: New file.
* test/lisp/erc/resources/base/commands/motd.eld: New file.