More than one resource invalidatiing breaks dataset edit form

[icmurray]

When attempting to add more than one resource at once, if more than one resource invalidates, this results in a js error, leaving the form in an inconsistent state.

Repro:

1. Go to /dataset/new
2. Add a new resource. Fill in one of the fields with an invalid value. eg - last_modified, or size...
3. Add another resource, doing the same thing: make one of the fields invalid.
4. Try to save the dataset.
5. The entered resource information will be lost, and a js error "Uncaught Error: Can't add the same model to a set twice,: backbone.js:586" will be thrown.