RFC: Add in special case handler for bootstrap errors

This patch adds in some really rudimentary exception printing when something comes up during bootstrap.  This should make the `fatal: error thrown and no exception handler available` errors a little less cryptic, as the actual problem is always hidden by Julia freaking out that we haven't given her an error handler yet.

@JeffBezanson, this is my first time mucking around with all these strange `jl_*` types; anything I majorly screwed up?