Stepping back away from this very familiar requirement a few paces, here are some things that worked for me:

1. On the server side, use a package such as one of the RPC::Any family as a convenient framework and protocol for dealing with requests including any that result in a server-side error.   (Whether an error occurs or not, the server’s response should always be a JSON packet.   The suggested package-suite neatly deals with this any many other sundry matters.   Click on the link next to the author’s name to see everything in it.)
2. On the client side, have the client include a short random string that the server is always obliged to echo back verbatim in its response, whatever that response may be.   The client side software checks it.
3. As a design point, I find it convenient that, if the server returns an error response, the client side initially throws an exception of its own, and catches it elsewhere.