Send any welcome error messages to the downstream

Currently, if we fail connecting to a new networking during welcome, we
send no error message to the client, and the connection remains open in
an undefined state.

Given the input:

  NICK nick
  USER user/invalid.xyz s e r
  PASS pass

soju will fail to connect, add a message to its own logs, but will
return no message to the downstream.

This fixes the issue by forwarding the error message if it is an IRC
error message (which it is for connecting to new networks).

We should probably also close the connection after the message is
written, because it leaves the connection in an undefined state. This is
TODO for now because we'd have to wait for the error message to be
written out first, which is non-trivial.

user.go

@@ -632,7 +632,18 @@ func (u *user) run() {
 			}
 
 			if err := dc.welcome(context.TODO()); err != nil {
+				if ircErr, ok := err.(ircError); ok {
+					msg := ircErr.Message.Copy()
+					msg.Prefix = dc.srv.prefix()
+					dc.SendMessage(msg)
+				} else {
+					dc.SendMessage(&irc.Message{
+						Command: "ERROR",
+						Params:  []string{"Internal server error"},
+					})
+				}
 				dc.logger.Printf("failed to handle new registered connection: %v", err)
+				// TODO: close dc after the error message is sent
 				break
 			}