Log unexpected exceptions raised by Conduit calls
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