Make validation messages on New URL consistent

The form labels this field as 'Redirects to', but the validation messages were
referring to it as 'New URL'. 'Redirects to is not a URL' reads a little
awkwardly, so let's make it consistent with existing validation messages on
bulk add.
Merge pull request #331 from alphagov/make_validation_messages_on_new_url_more_consistent

Make validation messages on New URL consistent