The behavior if no document chosen during adding extraction task is not univocal

[DaryaGurtovenko]

The reason of creation this issue is univocal behavior when user try to create new extraction job before any document(s) are chosen. The error which appears doesn't explain what should be done (even the document list is named 'Document name' but not 'file'). The error screen :

Steps to reproduce:

1. Open new extraction job page (via 'Add extraction' button on Extractions page or by direct link http://dev1.badgerdoc.com/extractions/add)/
2. Don't choose any documents. Press 'Next' button.
3. Correctly fill all mandatory fields.
4. Press 'Add extraction' button.

Result: The pop-up with the following error occurs: files and datasets cannot be empty at the same time. In dev console error 422 Unprocessable Entity.

Also it's impossible to save a draft without chosen documents.

1. Text of the error message may be change to direct order to make 'Choose any document(s) before job creation'.
2. Button 'Next' may be disabled if no document is chosen.
3. Have no idea about dataset so some solution includes dataset.

[SashaGraves]

Can reproduce: On the Main page, go to Jobs => click "New job" button => on Document tab click 'Next' without choosing any document => choose pipeline with extraction, add job name => click 'New Job' or 'Save a draft'

Result: cannot save