tests/test-gendoc-zh_TW.t

rust-dirs-multiset: improve temporary error message

While we wait on a future patch that could verify that the paths passed to
`DirsMultiset` have been audited, we still need to handle this error.
This patch makes it easier to bubble up and makes the error clearer.

Also, this patch introduces the `subslice_index` function that could be useful
for other - albeit niche - purposes.

Differential Revision: https://phab.mercurial-scm.org/D7921

#require docutils gettext

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