Fix JRuby random backtrace issue
Fix JRuby random backtrace issue

It appears an unreascued exception in a JRuby thread will be printed
out when the whole process exits. Rescuing and re-raising fixes it.

Also add a note about why this is in a thread to begin with (JRuby).
It wasn't obvious, but the answer was in the commit history.

expectedbehavior/instrumental_agent