.arcconfig

dispatch: quote filename in IOError as well

It's explicitly added for OSError at 720308f741cb "dispatch: show empty
filename in OSError aborts". Let's do the same for IOError.

{
    "conduit_uri": "https://phab.mercurial-scm.org/api",
    "repository.callsign": "HG",
    "arc.land.onto.default": "@",
    "base": "hg:.^"
}