Error Samples dont confirm to guidance

[JustABearOz]

The guidance in error-handling.md state 2 different http status codes, 400 for 'Not well-formed JSON' and 422 for 'Validation Errors'. The sample directly below this is for a 400 with validation errors. This seems to go directly against the guidance directly above!.

I'd suggest either changing the guidance to allow 400s for validation errors (probably more widely used in industry) or change the example to be a 422 😁