pytype: only output the "pytype crashed" message on error
If pytype did not crash while generating stub, that message is kind of
confusing. It seems simple enough to avoid it in this case.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}