if a substituter is entirely offline and cannot be queries at all we
should not be failing if other substituters are configured. likewise
if a substituter goes offline after querying but before we try using
it we should attempt to fetch that path from some other substituter.
ideally we'd treat all substituters as a single entity instead of as
one store each, then have that single entity take care of fallbacks,
retries, error reporting, etc. that requires larger rewrites though.
fixes#1061
Change-Id: I9d8fc0544ff380bf017256e8fcc82823dc634f10
eldritch horrors
879b07fd3d