add methods for mapping error responses

[robjtede]

PR Type

Feature

PR Checklist

• [ ] Tests for the changes have been added / updated.
• [ ] Documentation comments have been added / updated.
• [ ] A changelog entry has been made for the appropriate packages.
• [ ] Format code with the latest stable rustfmt.
• [x] (Team) Label with affected crates and semver status.

Overview

Give ability for users, middleware in particular, to alter the response generated from Errors.