.arcconfig

wireprotov2: add support for more response types

This adds types to represent error and generator responses from
server commands.

Differential Revision: https://phab.mercurial-scm.org/D3388

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