chainsaw-update: exit early if one of the intermediate command fails
That will prevent the user to be presented with a start that pretend to be
consistent with the request, but is not.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}