refactor(core): Move connection errors into their own file

packages/moxxmpp/lib/src/connection_errors.dart

@@ -0,0 +1,28 @@
+import 'package:moxxmpp/src/errors.dart';
+import 'package:moxxmpp/src/negotiators/negotiator.dart';
+
+/// The reason a call to `XmppConnection.connect` failed.
+abstract class XmppConnectionError {}
+
+/// Returned by `XmppConnection.connect` when a connection is already active.
+class ConnectionAlreadyRunningError extends XmppConnectionError {}
+
+/// Returned by `XmppConnection.connect` when a negotiator returned an unrecoverable
+/// error. Only returned when waitUntilLogin is true.
+class NegotiatorReturnedError extends XmppConnectionError {
+  NegotiatorReturnedError(this.error);
+
+  /// The error returned by the negotiator.
+  final NegotiatorError error;
+}
+
+class StreamFailureError extends XmppConnectionError {
+  StreamFailureError(this.error);
+
+  /// The error that causes a connection failure.
+  final XmppError error;
+}
+
+/// Returned by `XmppConnection.connect` when no connection could
+/// be established.
+class NoConnectionPossibleError extends XmppConnectionError {}