Add error message for unknown errors

Currently the `fail` option sets `file.error = true` when an unknown error occurs (eg: restart the web server during upload) so it is not possible to change or translate the error message, and in any case true was not a very good error message :)
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Merge pull request #3258 from tkleinhakisa/master

Add error message for unknown errors
Minor error message release version.
Merge branch 'master' of https://github.com/blueimp/jQuery-File-Upload