This:
- Consistently returns `nullptr` for a non-existent
store path, instead of a mix of `nullptr` and
throwing exceptions.
- If a store returns "bad" store paths in response
to a request (e.g. incorrect hash or name), don't
cache this result. This removes some duplication
of code at the cache-access layer of queryPathInfo()
checking this, and allows us to provide more
specific errors.
Part of #270.
Change-Id: I86612c6499b1a37ab872c712c2304d6a3ff19edb
julia
0fa289f559