Orinal bug title:
UpWiz should provide meaningful error message if file upload failed because an empty request was issued

Empty requests are issued, for example, if the browser cannot read the file due to missing read permission (but you can select them if you have the enumerate-files-permission for that folder).

The result of the request is also empty. This triggers a JavaScript error at https://github.com/wikimedia/mediawiki-extensions-UploadWizard/blob/6b1ba63356be72188bf9fb95f6d165381ee8f9de/resources/mw.UploadWizardUpload.js#L175
where result.error is undefined.

This makes UpWiz "spinning" foreever without telling the user that something went wrong.

Tested with Firefox 23 and Windows 7.

Version: unspecified
Severity: normal