when keys not found in redis, return NotFoundError

[deitch]

The file and memory driver distinguish between actual errors and things that are not found by returning a common.NotFoundError, which the admin handler check for.

The redis driver does not; for everything that it fails to get, it just returns an error.

The net result, e.g, is if you look up a certificate for a device that does not exist, with the memory or file driver, adam will return a 404, while the redis driver will return a 400, which is incorrect. Downstream things like eden then think it is an error, rather than something just not found yet.

This adds NotFoundError to places in redis where HGet and its variants return redis.Nil, which means, "I could not find the key."

[giggsoff]

I am not sure, but will we hit the same problem with calls to readCert function for example in DeviceGet?