Fix abrupt failure mode for uncloned repositories in Diffusion

Summary:
Fixes T9080. We try to list alternatives for the current ref (for example, if you're viewing a branch named "master" but there's also a tag named "master", or, in Mercurial, there are several branches named "master") but fail to abruptly if we can't get the list.

It's fine if we can't get the list; just continue. This is common when the repository hasn't cloned yet.

Test Plan: In a local repository with bad credentials, tried to do anything before and after. Before: completely blocked by error; after: things work normally.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9080

Differential Revision: https://secure.phabricator.com/D14044