Distinct error codes

[immesys]

The exit code for attempting to get a key that does not exist, and attempting to get a key from a client that does not exist (e.g. wrong port) are the same, and I think this is true for all operations. Having a distinct exit code for when you are unable to connect to the KV store as opposed to the key not existing would be very useful.

In particular ceph-docker uses kviator to wait for a key to appear indicating setup is complete. At present if the wrong port is used it waits forever, but I cannot fix this behaviour until kviator gives me a portable indication. I cannot also compare on the error text because it differs between KV stores.