400/422 error reporting of ISBN normalization API

Overview

The current implementation of the ISBN normalization APIs report errors using a 400 http status code and a JSON error description.

This violates FOLIO's standard where

• a 400 status code is used with a plain text error description
• a 422 status code is used with a JSON error description.

See the rtypes for examples for 400: https://github.com/folio-org/raml/tree/master/rtypes
See the validation trait for 422: https://github.com/folio-org/raml/blob/master/traits/validation.raml/validation.raml

See comment from original pull request - https://github.com/folio-org/mod-inventory/pull/123#discussion_r301029669

Acceptance Criteria

• Response codes are updated (RAML/etc.)
• Unit tests are updated