logger: Produce traces for all Throwables

logger: Produce traces for all Throwables

The code was checking for `Exception` to decide whether to produce traces,
so it wasn't providing them for PHP 7's new Errors. The code itself
works fine with any Throwable.

We have to keep parallel checks for Exception too for the time being,
because HHVM as used in Wikimedia production doesn't have Throwable.

Bug: T187147
Change-Id: Iec8a6718beb7ec09e45e332ee5762d0644ce17ab