fix w2fields test page: use a 'same server' URI to request the list ...

... (uri = /api/enum/user) so that the code does not fail immediately due to CORS restrictions anymore: the code does not always run on the w2ui.com server ;-)

Note: this test also includes an example of the new w2field onError handlers as introduced in commit 19af1c262f7b7322a88dbd801d332f83c7245293 / issue #409