fix(xmpp): clear any last strophe errors on reconnect

If user experiences any failed reconnect during the entire
session, even if followed by a success, then
the "connectionFailed" flag will trigger
JitsiConnectionEvents.CONNECTION_FAILED on hang up while
processing the disconnected transition.

modules/xmpp/xmpp.js

 
             logger.info(`My Jabber ID: ${this.connection.jid}`);
 
-            this.lastErrorMsg = undefined;
+            // XmppConnection emits CONNECTED again on reconnect - a good opportunity to clear any "last error" flags
+            this._resetState();
 
             // Schedule ping ?
             const pingJid = this.connection.domain;