StubServerSettings allows the server configuration file to specify
the default error to use for requests to the “error” app_id.
Requests to endpoints associated with this client will return the
error string.
Settings for StubRouter
These can be specified externally by the calling “client” during
registration and contain values that will be echoed back.
The server_credentials block is a JSON structure that contains
the default response for any routing request. This defaults to
‘error’.