- remove AllBlocked scheduler return code.  Nobody owned up to having
  created it or even knowing what it was there for.

- clean up fatal error condition handling somewhat.  The process
  exit code from a GHC program now indicates the kind of failure
  for certain kinds of exit:

	general internal RTS error	254
	program deadlocked		253
	program interrupted (ctrl-C)	252
	heap overflow			251
	main thread killed		250

  and we leave exit codes 1-199 for the user (as is traditional at MS,
  200-249 are reserved for future expansion, and may contain
  undocumented extensions :-)