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
$ hg init
$ echo a > a
$ hg ci -Ama
adding a
$ hg an a
0: a
$ hg --config ui.strict=False an a
0: a
$ echo "[ui]" >> $HGRCPATH
$ echo "strict=True" >> $HGRCPATH
$ hg an a
hg: unknown command 'an'
(use 'hg help' for a list of commands)
[255]
$ hg annotate a
0: a
should succeed - up is an alias, not an abbreviation
$ hg up
0 files updated, 0 files merged, 0 files removed, 0 files unresolved