Move non-success handling out of helper
Move non-success handling out of helper

The helper function handles both tests that pass, and tests that are
expected to fail (either by raising an exception or returning a redirect
to the login page).

By moving the handling of cases which are expected to fail out of the
helper function we can make the helper function less complex, which will
make further refactoring easier.

alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin