R1:b9a1260ef5c0

Improve top-level fatal exception handling in PHP 7+

Summary:
Depends on D20137. Ref T13250. Ref T12101. In versions of PHP beyond 7, various engine errors are gradually changing from internal fatals or internal errors to `Throwables`, a superclass of `Exception`.

This is generally a good change, but code written against PHP 5.x before `Throwable` was introduced may not catch these errors, even when the code is intended to be a top-level exception handler.

(The double-catch pattern here and elsewhere is…