Buttons interrupt file uploading

When creating or updating a dataset, if I'm waiting for a file to upload to the FileStore and I click one of the buttons _Add_ (when updating a dataset) or _Save & add another_ or _Next: Additional Info_ (when creating a new dataset) this silently interrupts the file upload and submits the form as if I'd never tried to upload the file.

This will result in an error _You must add at least one data resource_ if this is the dataset's first resource, or in silent failure if it's not the first resource (see <https://github.com/okfn/ckan/issues/901>). 

If someone tries to leave the page when an upload is in progress, they should get some sort of message like "File is still uploading. Abort?"