Invalid form entries should show errors in (retried) form, NOT redirect. (Issue #129)

This is what happens when you take an API designed for JSON and plop it in the middle of a traditional Web app; some assembly is required.  In this PR, we iterate adapting our existing action classes and controllers so that form-entry errors are displayed *in the form* where they can be corrected, rather than whisking the user away to a point where she has to start all over again.