Only allow the remote default branch checking to fail when the remote
default branch doesn't exist (`git_remote__default_branch` returns
`GIT_ENOTFOUND`). If there was any other type of error - for example, an
allocation failure - we should not swallow that and continue to fail.

This allows us to support the case when a remote has not advertised a
HEAD -- this is possible, for example, when the remote has constrained
the caller to a particular namespace. But other remote failures remain
as failures.