Resolves #254 Missed entities

lib/entity_widgets/missed_entity.dart

@@ -0,0 +1,19 @@
+part of '../main.dart';
+
+class MissedEntityWidget extends StatelessWidget {
+  MissedEntityWidget({
+    Key key
+  }) : super(key: key);
+
+  @override
+  Widget build(BuildContext context) {
+    final EntityModel entityModel = EntityModel.of(context);
+    return Container(
+        child: Padding(
+          padding: EdgeInsets.all(5.0),
+          child: Text("Entity not available: ${entityModel.entityWrapper.entity.entityId}"),
+        ),
+        color: Colors.amber[100],
+    );
+  }
+}