Re-render speed tagging interface on update errors

This changes the behaviour so that instead of rendering the "edit"
form when a user submits changes with validation errors from the
speed-tagging interface, they are shown the speed-tagging interface
again.
Merge pull request #1326 from alphagov/remain-in-speed-tagger-after-errors

Re-render speed tagging interface on update errors

https://www.pivotaltracker.com/story/show/67236308