Indicate retry on 412 Precondition Failed

[Joel Sing]

Under various situations (for example, setting the commit message or status on a merge proposal), we can get a 412 Precondition Failed response, which likely indicates that the underlying object was modified while we were trying to change it (and the eTag no longer matches).

We should either detect this case and return an appropriate error that indicates that the caller should refetch and retry the change (we probably want to avoid doing this ourselves, as we could overwrite the other change).