monitor: raise error when 'pretty' option is used with HMP

This is only semantically useful for QMP.

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
diff --git a/monitor/monitor.c b/monitor/monitor.c
index 640496e..636bcc8 100644
--- a/monitor/monitor.c
+++ b/monitor/monitor.c
@@ -716,8 +716,8 @@
             return -1;
         }
         if (opts->pretty) {
-            warn_report("'pretty' is deprecated for HMP monitors, it has no "
-                        "effect and will be removed in future versions");
+            error_setg(errp, "'pretty' is not compatible with HMP monitors");
+            return -1;
         }
         monitor_init_hmp(chr, true, &local_err);
         break;