Show helpful messages on project save errors
- Add a generic modal template and view
- Show the modal on error, with different messages based on the status code
Handle 404s in the project error messages.
Allow status and public fields to be blank, since they have defaults
Derive IsOwnerOrReadOnly from IsAuthenticatedOrReadOnly to cover the non-object-specific case
Use the complete set of Projects as the queryset on PUT/PATCH
Set href on error modal signin link
Merge pull request #5 from openplans/error-messages

Error messages saving projects