Errors generated by the `invisible-catcha` gem are reported as
`flash[:error]` (which differs from Rail's usual `flash[:alert]`).
We probably shouldn't use flash[:error] in our own codebase; but we can
handle its use in third-party libraries.
Errors generated by the `invisible-catcha` gem are reported as
`flash[:error]` (which differs from Rail's usual `flash[:alert]`).
Avoid crashing when the flash level passed to this helper is not known.
