Host Setup: Clear the clone error before starting a clone

One could have a clone that failed and then we're trying again with some
different values. The old error message is invalid.

lib/screens/githostsetup_screens.dart

@@ -350,6 +350,10 @@ class GitHostSetupScreenState extends State<GitHostSetupScreen> {
   }
 
   void _startGitClone(BuildContext context) async {
+    setState(() {
+      gitCloneErrorMessage = "";
+    });
+
     var appState = StateContainer.of(context).appState;
     var basePath = appState.gitBaseDirectory;