Improve error messsage of Jitsi widget

If openidtoken request fails, show
an error. Also render error a bit nicer
in generic widget load errors.

src/vector/jitsi/index.html

@@ -11,7 +11,9 @@
         <div class="joinConferencePrompt">
             <!-- TODO: i18n -->
             <h2>Jitsi Video Conference</h2>
-            <button type="button" id="joinButton">Join Conference</button>
+            <div id="widgetActionContainer">
+                <button type="button" id="joinButton">Join Conference</button>
+            </div>
         </div>
     </div>
 </div>