provide more specific error responses when submitting a new language code questionnaire

[da1nerd]

@vleong2332 @Redmayspecial:

Right now the api at http://td-demo.unfoldingword.org/api/questionnaire/ will respond with a success message or an error message. I would like to distinguish between the submittion failing due to a duplicate submission or some other error.

Allowed Response Status':

• success
• duplicate
• error

The addition of the duplicate status will allow the app to reliably determine if a questionnaire has already been submitted and update locally stored data as appropriate.
Explicitely stating the status as duplicate will eliminate possible bugs in parsing the message due to changes in the database schema, or format of the status message.

Response Structure

{
  "status": "success|duplicate|error",
  "message": "Some message"
}