Fix room directory maintaining and error state

Fixes https://github.com/vector-im/riot-web/issues/11665

We weren't clearing the error from a previous lookup, which effectively prevented the UI from working after an error happened.

src/components/structures/RoomDirectory.js

@@ -270,6 +270,7 @@ module.exports = createReactClass({
             roomServer: server,
             instanceId: instanceId,
             includeAll: includeAll,
+            error: null,
         }, this.refreshRoomList);
         // We also refresh the room list each time even though this
         // filtering is client-side. It hopefully won't be client side