Harmonise the Store::queryPathInfoUncached interface

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

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