Support `expose` property of http-errors

# Description/Steps to reproduce

As brought up in strongloop/loopback-next#1598, `strong-error-handler` should be able to support `expose` property set in errors thrown with `HttpError` from [`http-errors` module](https://www.npmjs.com/package/http-errors).

Its interaction with the `debug` setting in strong-error-handler is yet to be decided.

Read the error handler's existing debug behavior and the http-errors module documentation for its expose property first. The scope is complete when errors created with http-errors are supported and the interaction between expose and debug is explicitly decided and covered by verification.

Written by the indexing model from the issue text.