qapi: Smooth another visitor error checking pattern

Convert

    visit_type_FOO(v, ..., &ptr, &err);
    ...
    if (err) {
        ...
    }

to

    visit_type_FOO(v, ..., &ptr, errp);
    ...
    if (!ptr) {
        ...
    }

for functions that set @ptr to non-null / null on success / error.

Eliminate error_propagate() that are now unnecessary.  Delete @err
that are now unused.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200707160613.848843-40-armbru@redhat.com>
diff --git a/monitor/monitor.c b/monitor/monitor.c
index 19dcb8f..b385a3d 100644
--- a/monitor/monitor.c
+++ b/monitor/monitor.c
@@ -657,25 +657,18 @@
 {
     Visitor *v;
     MonitorOptions *options;
-    Error *local_err = NULL;
+    int ret;
 
     v = opts_visitor_new(opts);
-    visit_type_MonitorOptions(v, NULL, &options, &local_err);
+    visit_type_MonitorOptions(v, NULL, &options, errp);
     visit_free(v);
-
-    if (local_err) {
-        goto out;
-    }
-
-    monitor_init(options, true, &local_err);
-    qapi_free_MonitorOptions(options);
-
-out:
-    if (local_err) {
-        error_propagate(errp, local_err);
+    if (!options) {
         return -1;
     }
-    return 0;
+
+    ret = monitor_init(options, true, errp);
+    qapi_free_MonitorOptions(options);
+    return ret;
 }
 
 QemuOptsList qemu_mon_opts = {