Add exceptq support

[Gregg Young]

The trap should not occur, even with slow access. Unfortunately, the trap is not within fm/2 code, so it's hard to analyze without a process dump. I will be adding exceptq support to fm/2, once I get some in-progress exceptq rework completed. This will provide enhanced trap logging, without the need to resort to trap dumps.

[Steven Levine]

This is code complete, unless something shows up during stress testing.