Add APIError flask error handler
Add APIError flask error handler

This is modelled after the similar change in the supplier frontend

https://github.com/alphagov/digitalmarketplace-supplier-frontend/commit/233f8840d55cadb9fb7fe60ff12c53b0f59f23a5

alphagov/digitalmarketplace-admin-frontend,alphagov/digitalmarketplace-admin-frontend,alphagov/digitalmarketplace-admin-frontend,alphagov/digitalmarketplace-admin-frontend