dispatch: move IOError handling and flushing of streams to `dispatch()` Instead of patching both dispatch code and commandserver code, we directly handle this in `dispatch.dispatch()`. Thanks to Yuya who recommended this.

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