UI issues for failed uploads

[trepmal]

When uploading a file with a forbidden extension using the multi-file uploader, the upload will (of course) fail. But the "crunching" bar remains visible, and if the Gallery tab is visible, the attachment count is wrongly incremented.

Will also happen if there are directory permission issues ('cannot write to directory', 'uploaded file could not be moved').

Screencast showing gallery tab incrementing: ​http://screencast.com/t/EJtWF2kXXBki

Although the correct error messages are sent from async-upload.php, the js (wp-includes/js/plupload/handlers.js) looks for a particular string to check if the response is an error, and this string is not present.

Patch adds the string as an easy fix.

Could probably as well check for error-div in handlers.dev.js.

In 3.2, the progress bar was hidden in prepareMediaItem() (this was changed in [19346]):
http://core.trac.wordpress.org/browser/tags/3.2.1/wp-includes/js/swfupload/handlers.dev.js#L48
The Gallery tab counter was still incrementing though.

In [20580]:

Uploader: fix check for error message send from async-upload on failed uploads, improve styling it a bit, props trepmal, SergeyBiryukov, fixes #20445