Enable custom reject implementation to leverage the built-in "code to statusCode" mapping

[bajtos]

Users wishing to provide their own implementation of reject action need a way how to decide which HTTP status code to use depending on the error code, e.g. to map error code ENTITY_NOT_FOUND to status code 404.

See also the discussion in #1867

Acceptance criteria

• A public API allowing custom reject implementations to map error codes to status codes
• A new subsection in Handling errors explaining how to write a custom reject implementation