Suppress CORS errors in the 'failed to join room' dialog

This isn't pretty, but it does address https://github.com/vector-im/riot-web/issues/7526

Tested with Chrome and Firefox to ensure the message is stable enough to parse.

src/stores/RoomViewStore.js

@@ -224,6 +224,9 @@ class RoomViewStore extends Store {
                 err: err,
             });
             let msg = err.message ? err.message : JSON.stringify(err);
+            if (msg && msg.startsWith("CORS request rejected")) {
+                msg = _t("There was an error joining the room");
+            }
             if (err.errcode === 'M_INCOMPATIBLE_ROOM_VERSION') {
                 msg = <div>
                     {_t("Sorry, your homeserver is too old to participate in this room.")}<br />