Add human readable error messages

[61131]

While earlier versions of this code included human readable error messages, this was removed while aspects of the code layout was rearranged for ease of development. Once feature development has been completed, human readable error messages should be reintroduced such that clients can be notified as to the reason for unsuccessful requests.

As per RFC 8951, these human readable error messages should be returned with a content type of "text/plain" to minimise confusion regarding the interpretation of these messages.