[racket] DrRacket locking up on errors in non-GUI worker threads

Also, I should add that returning snips from the user's program and
then having DrRacket call their callbacks (for drawing and sizing and
other things) is actually a bug in DrRacket; it is a security hole. So
far I have not closed it but I really should.

I think that maybe the way to close it is to build a new kind of proxy
snip that proxies to the untrusted snip back in the user's space but
does something reasonable when the original snip's callbacks go wrong
or when the user's eventspace main thread is killed.

I'll move that higher up on the list.

Robby