tests/test-gendoc-el.t

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

#require docutils gettext

  $ $TESTDIR/check-gendoc el
  checking for parse errors