Provide codified error responses in placement

This blueprint aims to extend the placement API to include detailed, consistent error messages in the response body to allow clear distinctions between different errors that use the same HTTP status code.

Error conditions in the placement REST API are signalled by HTTP status codes
and message bodies. For some of the errors it is impossible to distinguish the
reason why a particular response was issued other than reading the message
which was returned in the response body. This approach creates a strong
coupling between client code and the exact message of the error response. This
is very fragile in the face of change.

We propose to extend existing HTTP error responses with a ``code`` field as
described in the API-WG Errors_ specification. This will help consumers of the
API to easily recognize the type of the error cases. Note that the new format
of the response is for ``application/json`` only.