usernode.module sabotages all drupal_set_message() output during node creation ...

... and that's wrong. It's fine to suppress the message 'usernode created successfully', but when the usernode is *not* being created successfully -- http://drupal.org/node/263507 -- then you are throwing away important information!

the solution would be to drupal_get_messages, then do the work, then drupal_get_messages() again, check whether the result is just your one unwanted error, and if not, combine the new error with the previous existing errors.