Expose fault information to unwind handlers

[nwf]

The switcher's exception path goes through some significant effort to inform the fault handlers of what happened. However, that information is discarded entirely in the unwind.h machinery and its stackless handler implementation.

1. It would be ideal if the fault information could be plumbed through to the the unwind handlers.

2. While the stackless handlers cannot, by necessity, be granted a view of the entire register file, they nevertheless could, but are not, provided with a view of the faulting PCC, which is among the most useful information in an error report (especially while we are without a debugger).