Collect errors from stepper for Proposal validation

[miggol]

It's (hopefully) time to retire validate_proposal.py!

The stepper.items list already contains all the modelform items that a proposal requires to be completed. Each has an item.errors property that will allow you to access the form errors.

Implementing this may require slightly more work than just glueing everything together. There could be quirks in validate_proposal.py and errors that get generated elsewhere than in modelform instantiation. Specifically I hope that #691 doesn't rear its head.