qapi: Only input visitors can actually fail

The previous few commits have made this more obvious, and removed the
one exception.  Time to clarify the documentation, and drop dead error
checking.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200424084338.26803-13-armbru@redhat.com>

include/qapi/visitor-impl.h

@@ -43,6 +43,10 @@ typedef enum VisitorType {
 
 struct Visitor
 {
+    /*
+     * Only input visitors may fail!
+     */
+
     /* Must be set to visit structs */
     void (*start_struct)(Visitor *v, const char *name, void **obj,
                          size_t size, Error **errp);