ui: Improve "change vnc" error reporting

Switch from monitor_printf() to error_setg() and hmp_handle_error().
This makes "this is an error" more obvious both in the source and in
the monitor, where hmp_handle_error() prefixes the message with
"Error: ".

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20230109190321.1056914-13-armbru@redhat.com>

monitor/hmp-cmds.c

@@ -1097,9 +1097,8 @@ void hmp_change(Monitor *mon, const QDict *qdict)
 #ifdef CONFIG_VNC
     if (strcmp(device, "vnc") == 0) {
         if (read_only) {
-            monitor_printf(mon,
-                           "Parameter 'read-only-mode' is invalid for VNC\n");
-            return;
+            error_setg(&err, "Parameter 'read-only-mode' is invalid for VNC");
+            goto end;
         }
         if (strcmp(target, "passwd") == 0 ||
             strcmp(target, "password") == 0) {
@@ -1111,7 +1110,8 @@ void hmp_change(Monitor *mon, const QDict *qdict)
                 qmp_change_vnc_password(arg, &err);
             }
         } else {
-            monitor_printf(mon, "Expected 'password' after 'vnc'\n");
+            error_setg(&err, "Expected 'password' after 'vnc'");
+            goto end;
         }
     } else
 #endif