R1:c65b58b21c32

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…