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