title: Django unique constraint + form errors
body: <p>I'm having some issues with double-posting on my site. I figure a simple unique constraint across all the relevant fields will solve the issue on a database level, but then it just produces a nasty error page for the user. Is there a way I can turn this into a pretty form error instead? Like a <code>non_field_error</code>? Or what approach should I take?</p>