rhg: Propagate permission errors when finding a repository The Rust standard library has a `Path::is_dir` method that returns false for any I/O error (such as a permission error), not just "No such file or directory". Instead add an `is_dir` function that returns false for non-directories and for "No such file or directory" errors, but propagates other I/O errors. Differential Revision: https://phab.mercurial-scm.org/D11230