dovecot - Sep 2017 - Returning error response to the client from passdb (with dict protocol)

Hi,

When using a passdb with (UNIX socket) dict protocol, I can return a 
JSON with fields. When I want to deny the user, I can send a JSON like this:

{"fail":true}

which yields (on POP) a friendly -ERR [AUTH] Authentication failed.

Is it possible to return the response text itself? I want to tell the 
user why the authentication has failed.