Use correct exception when a record can not be found

In 4a505f1b the "nextstatus"
code was replaced by raising an exception which is handled by OBS
and causes a 404 view to be rendered.
But since ActionController::RoutingError exceptions also indicate
a bug in OBS we also notify errbit, our exception tracker, about
such errors.
This was causing our exception tracker to be spammed.

Instead we now send ActiveRecord::RecordNotFound exceptions, which
renders a 404 without notifying errbit.

Fixes #7080