GitHost: Improve error handling

In the case of OAuth errors which we get through the message
channel, throwing an exception doesn't help us as the init function
will not be aware about it.

So for now, I've moved the nodejs style error handling in this
particular case.

lib/apis/github.dart

@@ -30,11 +30,16 @@ class GitHub implements GitHost {
       var authCode = uri.queryParameters['code'];
       if (authCode == null) {
         print("GitHub: Missing auth code. Now what?");
-        throw GitHostException.OAuthFailed;
+        callback(GitHostException.OAuthFailed);
       }
 
       this._accessCode = await _getAccessCode(authCode);
-      callback();
+      if (this._accessCode == null || this._accessCode.isEmpty) {
+        print("GitHub: AccessCode is invalid: " + this._accessCode);
+        callback(GitHostException.OAuthFailed);
+      }
+
+      callback(null);
     }
 
     _platform.setMethodCallHandler(_handleMessages);

lib/apis/gitlab.dart

@@ -36,15 +36,17 @@ class GitLab implements GitHost {
         print("GitLab: OAuth State incorrect");
         print("Required State: " + _stateOAuth);
         print("Actual State: " + state);
-        throw GitHostException.OAuthFailed;
+        callback(GitHostException.OAuthFailed);
+        return;
       }
 
       _accessCode = map['access_token'];
       if (_accessCode == null) {
-        throw GitHostException.OAuthFailed;
+        callback(GitHostException.OAuthFailed);
+        return;
       }
 
-      callback();
+      callback(null);
     }
 
     _platform.setMethodCallHandler(_handleMessages);

lib/screens/githostsetup_autoconfigure.dart

@@ -29,7 +29,10 @@ class GitHostSetupAutoConfigureState extends State<GitHostSetupAutoConfigure> {
     print("Starting autoconfigure");
     gitHost = createGitHost(widget.gitHostType);
     try {
-      gitHost.init(() async {
+      gitHost.init((Exception error) async {
+        if (error != null) {
+          throw error;
+        }
         print("GitHost Initalized: " + widget.gitHostType.toString());
 
         GitRepo repo;