rhg: raise wdir specific error for `hg debugdata`

Helps remove the conditional in `test-debugcommands.t` for rhg.

Differential Revision: https://phab.mercurial-scm.org/D10254
14 jobs for topic/default/rust-rocket in 28 minutes and 18 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Tests
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phabricator-refresh

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