Error string and help for unsupported file type
Display error message if file type is invalid

Raise a custom exception type and rescue it in the controller,
returning JSON so the UI can display a sensible error message.
Display invalid file in upload error messages
New "upload_failed" translation string
zero margin so that the progress lines up properly

margin-left is set to 4px in ui.css, the clause needs to be as specific
in order to override it.
Show error messages inline during uploading

The old error message was displayed using the dialog's
error display.  It was out of the way at the top right
and easy to miss, plus it also only appeared for a few seconds.
Catch exception when parsing an invalid JSON reply

The server *should* always reply with JSON, but don't rely on it.
Create document in "error" state on invalid file