Correct {cli} markup for informative failure

[MichaelChirico]

c.f. current error we observe:

Error in `...`: Snapshot file names must be unique. `name` has already been used.

vs. now

Error in `...`: Snapshot file names must be unique. "pval-direction-both.svg" has already been used.

Introduced in #1592

[MichaelChirico]

I'm unable to update the snapshot myself at the moment. I tried the recommended new testthat::snapshot_download_gh() helper, but was rebuffed:

Error in `gh::gh()`:
! GitHub API error (403): Must have admin rights to Repository.
ℹ Read more at <https://docs.github.com/rest/actions/workflow-jobs#download-job-logs-for-a-workflow-run>
Run `rlang::last_trace()` to see where the error occurred.

(PS, it's not clear to me from the docs how to use dest_dir -- which directory should I be targeting, exactly? the package top-level directory? or should it point to the _snaps directory?)