While doing a boltdb operation and if the bucket is not found
we should not return a boltdb specific bucket not found error
because this causes leaky abstraction where in the user of libkv
needs to know about boltdb and import boltdb dependencies
neither of which is desirable. Replaced all the bucket not found
errors with the more generic `store.ErrKeyNotFound` error which
is more appropriate.

Signed-off-by: Jana Radhakrishnan <mrjana@docker.com>