Problem: Misleading error code in case ZAP handler sends an invalid status code (#2646)

Solution: Use EPROTO instead of EACCES error code in that case

src/curve_server.cpp

@@ -317,7 +317,7 @@ int zmq::curve_server_t::process_hello (msg_t *msg_)
                               hello_nonce, cn_client, secret_key);
     if (rc != 0) {
         // CURVE I: cannot open client HELLO -- wrong server key?
-        current_error_detail = encryption;
+        current_error_detail = encryption;
         errno = EPROTO;
         return -1;
     }
@@ -709,7 +709,7 @@ int zmq::curve_server_t::receive_and_process_zap_reply ()
     if (msg [3].size () != 3) {
         // CURVE I: ZAP handler sent invalid status code
         current_error_detail = zap;
-        errno = EACCES;
+        errno = EPROTO;
         return close_and_return (msg, -1);
     }