Determine guidance for top level error code mapping to HTTP status codes

[markweitzel]

Determine if top level error codes should map to HTTP status codes or be more granular. Provide a consistent set of guidance for our service team partners.

[johanste]

The OData v4 specification says:

The value for the code name/value pair is a language-independent string. Its value is a service-defined error code. This code serves as a sub-status for the HTTP error code specified in the response.

There is no strict requirement that we follow OData to a tee here (we are clearly taking liberties in other areas), but the rationale as to what the value represents should carry over.