R1:187356fea515

Let the top-level exception handler dump a stack trace if we reach debug mode before things go sideways

Summary:
Depends on D20140. Ref T13250. Currently, the top-level exception handler doesn't dump stacks because we might not be in debug mode, and we might double-extra-super fatal if we call `PhabricatorEnv:...` to try to figure out if we're in debug mode or not.

We can get around this by setting a flag on the Sink once we're able to confirm that we're in debug mode. Then it's okay for the top-level…