Display error messages

lib/main.dart

@@ -206,6 +206,21 @@ class _MyHomePageState extends State<MyHomePage> {
     });
   }
 
+  void _showMessage(String string, BuildContext context) {
+    showDialog<String>(
+      context: context,
+      builder: (BuildContext context) => AlertDialog(
+        content: Text(string),
+        actions: <Widget>[
+          TextButton(
+            onPressed: () => Navigator.pop(context, 'OK'),
+            child: const Text('OK'),
+          ),
+        ],
+      ),
+    );
+  }
+
   // Get coords between origin/dest points corresponding to shortest path.
   void _getDirections() async {
     _resetPolyline();
@@ -228,6 +243,18 @@ class _MyHomePageState extends State<MyHomePage> {
           }),
           headers: <String, String>{"Content-Type": "application/json"});
       _data = jsonDecode(response.body);
+
+      if (_data.containsKey('error_code') && _data['error_code'] != 0) {
+        switch(_data['error_code']) {
+        case 1:
+          _showMessage('The address is outside the solution space. Please enter an address within Berlin-Brandenburg.', context);
+          return;
+        case 2:
+          _showMessage('The address could not be found.', context);
+          return;
+        }
+      }
+
       var coordinates = _data['route'];
       int duration = _data['duration'];
       int distance = _data['distance'];