Remove tight java coupling

[necrophonic]

The error report should be replaced with a proper RFC7807 standards compliant problem report https://tools.ietf.org/html/rfc7807 - see https://github.com/ONSdigital/sdx-evolution/blob/master/lib/api/problem.go

Shouldn't be tied to implementation details of a framework being used in a separate service - at the least, services should respond to plain HTTP headers and codes as laid out in the specification and not complain if they don't receive specific structures.