[Compiler-sig] Error checking macros

[Jeremy Hylton]

I think the macros are probably a good idea here.  In general I don't
like macros that hide a return, because it obscures the control flow.
On the third hand, this is all generated code so it doesn't actually
matter what it looks like.

You've probably noticed, though, that I skipped error checking almost
completely in the astmodule.  I'm worried about how much longer the
code will be in *that* module.

Jeremy