In order to notify the user when we are disconnected from a network
(either due to an error, or due a QUIT), and when we fail reconnecting,
this commit adds support for sending a short NOTICE message from the
service user to all relevant downstreams.
The last error is stored, and cleared on successful connection, to
ensure that the user is *not* flooded with identical connection error
messages, which can often happen when a server is down.
No lock is needed on lastError because it is only read and modified from
the user goroutine.
Closes: https://todo.sr.ht/~emersion/soju/27
