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