R1:08126d39049e

Improve top-level exception handling

Summary:
Fixes T6692. Addresses two main issues:

- The write guard would sometimes not get disposed of on exception pathways, generating an unnecessary secondary error which was just a symptom of the original root error.
- This was generally confusing and reduced the quality of reports we received because users would report the symptomatic error sometimes instead of the real error.
- Instead, reflow the handling so that we always dispose of the write guard…