Add an error overlay.

Assinging to `error` on the Map componenent will cause an overlay
with an error <v-alert/> to be shown.

lib/www/client/source/src/views/Map.vue

@@ -1,5 +1,8 @@
 <template>
 <div>
+  <v-overlay :value="error">
+    <v-alert type="error">{{ error }}</v-alert>
+  </v-overlay>
   <div id="map">
   </div>
   <div> <!-- This div holds the map overlays -->
@@ -299,7 +302,9 @@ export default {
       searchText: "",
       searchVisible: false,
       filterText: "",
-      filterVisible: false
+      filterVisible: false,
+
+      error: null
     };
   },