Always seek() to beginning of StringIO obj, Fix #5062

Very small pull req for the amount of time it took to track it down.  =)

It seems that non-ascii characters *are* supported by both msgpack and YAML.  But somehow (possibly in the logging try-except at the bottom of the for loop where the change was made) the StringIO was getting read, but not reset.

I also added a log for the results of the YAML renderer while I was there, because we need to work on making it easier to debug YAML and other errors.