Refactor to use `render_error_page` to handle User::PrivilegeError
exceptions. This way these exceptions are logged to New Relic.
Changes:
* Anonymous users aren't automatically redirected to the login page.
Instead they're taken to the access denied page, which links to the
login/signup pages.
* JSON/XML error responses return `message` instead of `reason`.
c7f8fbbec2