[Stitching] Better errors

[alloy]

These changes will likely require that we change errors to be objects rather than just message strings, will have to look into whether or that’s actually supported according to the spec.

• Services and MP should start using HTTP status codes to reflect the type of error
• Services should generate UUIDs for errors and:
  • log locally the details associated with that error UUID
  • include that UUID in the error payload
  • include whether or not it is a public or private error message
• MP should only report the error UUID in case of a private error message
• MP should annotate the error object with the service that the error originated from