Make PAL capable of producing back traces.

[mjp41]

In #88, we implemented enough backtrace to flush out some CI failures for Windows. This should be refactored.

• We should always call Pal::error instead of abort.
• Assert should be refactored to call Pal::error in the failure case.
• In some configurations Pal::error should produce a backtrace.
• The Pal on Windows should account for running in headless CI, and stop generating dialog boxes.

[mjp41]

#161 this contributes to addressing this issue.