Clean up a ConduitException around Diffusion merges

Summary:
Ref T7123. Two general issues:

For proxied repositories, we currently throw a ConduitClientException, vs ConduitException for local repositories. This is inconsistent and we should fix it, but I also want to examine the use of try-the-call-and-throw at these sites since it may be something we can update. In particular, trying a call that we know will always fail is now more expensive (in proxied repositories) than it used to be.

Here, we try-and-throw for merges, but they're never supported in Subversion. Just don't bother trying.

Test Plan: Browsed a SVN repository with proxying set up, got a clean commit page.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7123

Differential Revision: https://secure.phabricator.com/D11646