Make ERC's error-notice formatting more consistent

Make ERC's error-notice formatting more consistent

* lisp/erc/erc-backend.el (erc--server-propagate-failed-connection):
Include leading three-asterisk notice prefix when reporting process
exit status, and set the `erc-msg' text property to `notice-error'.
(erc-schedule-reconnect): Include leading notice prefix when inserting
`reconnecting' message.
* lisp/erc/erc.el (erc-process-input-line, erc-cmd-PART,
erc-cmd-TOPIC): Display `no-target' messages as "error notices".
(erc-message-english-disconnected,
erc-message-english-disconnected-noreconnect): Hard-code standard
value of `erc-notice-prefix' into message text for consistency during
formatting and insertion.
* test/lisp/erc/erc-tests.el (erc--refresh-prompt): Expect notice
prefix before `no-target' message.  (Bug#67031)