In Rails 5 catches invalid UTF8 characters in querystring in a
params middleware, errors are handled by a params middleware and
raises a BadRequest exception. This means that these UTF8 errors
are not raised deeper in application stack and these can't also
be handled on application level.

If we would want to have custom handler for these errors, we would
have to create a new middleware and insert it before actionpack's
params middleware and rescue BadRequest exceptions there. But there
is no need to do this currently (see discussion on
https://gitlab.com/gitlab-org/gitlab-ce/issues/51908)