R1:56bcb33a1891

Improve exception reporting behavior for core exceptions

Summary:
See <https://github.com/facebook/phabricator/issues/487>. If an exception is thrown too high in the stack for the main exception handling to deal with it, we currently never report a stack trace. Instead:

- Always report a stack trace to the error log.
- With developer mode, also report a stack trace to the screen.

Test Plan: Added a high-level `throw` and hit both cases. Got traces in the log and traces-under-developer-mode on screen.