bouncer-networks: Add a read-only error attribute

This is useful for clients to display additional info abotu why a
network is disconnected.

doc/ext/bouncer-networks.md

@@ -242,6 +242,10 @@ Bouncers MUST recognise the following network attributes:
 * `realname`: the realname to use during registration.
 * `pass`: the server password (PASS) to use during registration.
 
+Bouncers MAY recognise the following network attributes:
+* `error` (read-only): a human-readable short text describing an error with the current network.
+  This is typically used when the bouncer state is `disconnected` to describe the reason why the bouncer is disconnected.
+
 TODO: more attributes
 
 ### Examples

downstream.go

@@ -142,6 +142,10 @@ func getNetworkAttrs(network *network) irc.Tags {
 		attrs["realname"] = irc.TagValue(realname)
 	}
 
+	if network.lastError != nil {
+		attrs["error"] = irc.TagValue(network.lastError.Error())
+	}
+
 	fillNetworkAddrAttrs(attrs, &network.Network)
 
 	return attrs

user.go

@@ -576,7 +576,10 @@ func (u *user) run() {
 					dc.SendMessage(&irc.Message{
 						Prefix:  dc.srv.prefix(),
 						Command: "BOUNCER",
-						Params:  []string{"NETWORK", netIDStr, "state=connected"},
+						Params: []string{"NETWORK", netIDStr, irc.Tags{
+							"state": "connected",
+							"error": "",
+						}.String()},
 					})
 				}
 			})
@@ -599,6 +602,17 @@ func (u *user) run() {
 				})
 			}
 			net.lastError = e.err
+			u.forEachDownstream(func(dc *downstreamConn) {
+				if dc.caps.IsEnabled("soju.im/bouncer-networks-notify") {
+					dc.SendMessage(&irc.Message{
+						Prefix:  dc.srv.prefix(),
+						Command: "BOUNCER",
+						Params: []string{"NETWORK", fmt.Sprintf("%v", net.ID), irc.Tags{
+							"error": irc.TagValue(net.lastError.Error()),
+						}.String()},
+					})
+				}
+			})
 		case eventUpstreamError:
 			uc := e.uc
 
@@ -606,6 +620,17 @@ func (u *user) run() {
 				sendServiceNOTICE(dc, fmt.Sprintf("disconnected from %s: %v", uc.network.GetName(), e.err))
 			})
 			uc.network.lastError = e.err
+			u.forEachDownstream(func(dc *downstreamConn) {
+				if dc.caps.IsEnabled("soju.im/bouncer-networks-notify") {
+					dc.SendMessage(&irc.Message{
+						Prefix:  dc.srv.prefix(),
+						Command: "BOUNCER",
+						Params: []string{"NETWORK", fmt.Sprintf("%v", uc.network.ID), irc.Tags{
+							"error": irc.TagValue(uc.network.lastError.Error()),
+						}.String()},
+					})
+				}
+			})
 		case eventUpstreamMessage:
 			msg, uc := e.msg, e.uc
 			if uc.isClosed() {