Response error message conventions/review #214
Description
The two transfer services share code for a consistent approach to error messages in responses, responding with a message string and additional details in a generic JSON field. They don't always use these consistently though, the messages vary in level of detail and the additional JSON can just be whatever. A good place to start would be making sure the message strings are always simple and suitable for presentation to end users, and that any (optional) technical details are in the JSON.
Should also review the content of error messages in cases where the text comes from the underlying FHIR servers. Sometimes returning these as-is is going to be undesirable (e.g. requesting a transfer via an ID that isn't a patient record leaks both whether that ID is in use AND what sort of record it identifies, which isn't necessarily something an end client of a transfer request needs to know).