feat: Message events now contain the stanza error, if available

packages/moxxmpp/lib/src/events.dart

@@ -63,6 +63,7 @@ class MessageEvent extends XmppEvent {
     required this.isMarkable,
     required this.encrypted,
     required this.other,
+    this.error,
     this.type,
     this.oob,
     this.sfs,
@@ -74,6 +75,7 @@ class MessageEvent extends XmppEvent {
     this.funCancellation,
     this.messageRetraction,
   });
+  final StanzaError? error;
   final String body;
   final JID fromJid;
   final JID toJid;

packages/moxxmpp/lib/src/message.dart

@@ -99,6 +99,7 @@ class MessageManager extends XmppManagerBase {
       encrypted: state.encrypted,
       messageRetraction: state.messageRetraction,
       other: state.other,
+      error: StanzaError.fromStanza(message),
     ),);
 
     return state.copyWith(done: true);

packages/moxxmpp/lib/src/stanza.dart

@@ -1,6 +1,28 @@
 import 'package:moxxmpp/src/namespaces.dart';
 import 'package:moxxmpp/src/stringxml.dart';
 
+/// A simple description of the <error /> element that may be inside a stanza
+class StanzaError {
+  StanzaError(this.type, this.error);
+  String type;
+  String error;
+
+  /// Returns a StanzaError if [stanza] contains a <error /> element. If not, returns
+  /// null.
+  static StanzaError? fromStanza(Stanza stanza) {
+    final error = stanza.firstTag('error');
+    if (error == null) return null;
+
+    final stanzaError = error.firstTagByXmlns(fullStanzaXmlns);
+    if (stanzaError == null) return null;
+
+    return StanzaError(
+      error.attributes['type']! as String,
+      stanzaError.tag,
+    );
+  }
+}
+
 class Stanza extends XMLNode {
   // ignore: use_super_parameters
   Stanza({ this.to, this.from, this.type, this.id, List<XMLNode> children = const [], required String tag, Map<String, String> attributes = const {} }) : super(