feat(conference): store conference errors in redux

react/features/base/conference/reducer.js

     return assign(state, {
         authRequired,
         conference: undefined,
+        error,
         joining: undefined,
         leaving: undefined,
 
  * reduction of the specified action.
  */
 function _conferenceWillJoin(state, { conference }) {
-    return set(state, 'joining', conference);
+    return assign(state, {
+        error: undefined,
+        joining: conference
+    });
 }
 
 /**