Log unexpected exceptions raised by Conduit calls

Summary:
Ref T13581. Currently, unexpected exceptions inside Conduit calls are passed to the client, but not logged on the server.

These exceptions should generally be unexpected, and producing a server-side trace is potentially useful.

Test Plan: Simulated a during-execution exception, saw it get logged on the server.

Maniphest Tasks: T13581

Differential Revision: https://secure.phabricator.com/D21464