fix(core): Fix crash when no negotiator matches

Fixes #30.

Also removes the `allowPlainAuth` attribute of `ConnectionSettings` as
users who want to disable SASL PLAIN can just not register the
negotiator or extend it.

packages/moxxmpp/lib/src/connection_errors.dart

@@ -2,16 +2,22 @@ import 'package:moxxmpp/src/errors.dart';
 import 'package:moxxmpp/src/negotiators/negotiator.dart';
 
 /// The reason a call to `XmppConnection.connect` failed.
-abstract class XmppConnectionError {}
+abstract class XmppConnectionError extends XmppError {}
 
 /// Returned by `XmppConnection.connect` when a connection is already active.
-class ConnectionAlreadyRunningError extends XmppConnectionError {}
+class ConnectionAlreadyRunningError extends XmppConnectionError {
+  @override
+  bool isRecoverable() => true;
+}
 
 /// Returned by `XmppConnection.connect` when a negotiator returned an unrecoverable
 /// error. Only returned when waitUntilLogin is true.
 class NegotiatorReturnedError extends XmppConnectionError {
   NegotiatorReturnedError(this.error);
 
+  @override
+  bool isRecoverable() => error.isRecoverable();
+
   /// The error returned by the negotiator.
   final NegotiatorError error;
 }
@@ -19,10 +25,30 @@ class NegotiatorReturnedError extends XmppConnectionError {
 class StreamFailureError extends XmppConnectionError {
   StreamFailureError(this.error);
 
+  @override
+  bool isRecoverable() => error.isRecoverable();
+
   /// The error that causes a connection failure.
   final XmppError error;
 }
 
 /// Returned by `XmppConnection.connect` when no connection could
 /// be established.
-class NoConnectionPossibleError extends XmppConnectionError {}
+class NoConnectionPossibleError extends XmppConnectionError {
+  @override
+  bool isRecoverable() => true;
+}
+
+/// Returned if no matching authentication mechanism has been presented
+class NoMatchingAuthenticationMechanismAvailableError
+    extends XmppConnectionError {
+  @override
+  bool isRecoverable() => false;
+}
+
+/// Returned if no negotiator was picked, even though negotiations are not done
+/// yet.
+class NoAuthenticatorAvailableError extends XmppConnectionError {
+  @override
+  bool isRecoverable() => false;
+}