We ought to harden the knotmirror in case we get errors from tap or anywhere else, in situations such that the appview knows about a git repo somewhere but the knot- mirror doesn't. I would call this good practice in general so that we have robust infrastructure and other possible future appviews would also be able to trust that the knotmirror will serve them repos that in fact exist. Lewis: May this revision serve well! <lewis@tangled.org>