Add an error component

Shows when a rendering error has been triggered, like the one in 5edc402a01

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

js/src/views/Event/Event.vue

@@ -1,6 +1,5 @@
 <template>
   <div class="container">
-    <b-loading :active.sync="$apollo.loading" />
     <transition appear name="fade" mode="out-in">
       <div>
         <div