rhg: Add more conversions between error types
This allows using the `?` operator in more places, as the next commit does.
Differential Revision: https://phab.mercurial-scm.org/D10238
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"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}