SI-5634 eliminate overly verbose error message

In fact I couldn't see where the error message described in the ticket is
being produced, and I'm supposing it has already vacated the premises. I went
through ClassfileParser and UnPickler, the two enclaves most likely to harbor
the spewer, and hardened the output. No printlns, tighter assertion messages,
and string interpolation for aesthetics. We're not savages.