Display POST errors on widget initialization

Call me silly, but I have a hard time remembering why the upload button is greyed out sometimes.

If the POST request to /swfupload (done by the javascript) returns an error, we should probably show it, don't you think?

Note: I tested this using jQuery 1.3 (jquery_update module 6.x-2.0 installed).
This is relevant since something may have changed to the jqXHR object in jQuery >= 1.4. If an interested person could test that...