Diffusion - clean up catching ConduitException

Summary: Ref T7123. Turns out that we might throw ConduitClientException now in proxied scenarios. For all but one callsite remove the try / catch bit and don't issue the call for SVN. For the remaining callsite, also don't issue the call for SVN but keep in the exception logic since its renders a pretty error message in the non-proxied case?

Test Plan: played around with diffusion and things looked okay.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7123

Differential Revision: https://secure.phabricator.com/D11789