clean up exception use in backend #30
Description
Some of the backend modules (asm, x86, ra, pe) use 'raise' or 'failwith' in a
way that escapes to the top level. This is undesirable; we have more structured
error handling in the middle-end, and can probably classify / manage such
errors a bit better. Though of course, nothing's ever "really" supposed to go
wrong in the backend...