CLI Generates Stack Trace when Run without Server

When running a `neptune-cli` command that interacts with the RPC server, but without simultaneously running a `neptune-core` instance, the connection will obviously fail but `neptune-cli` will spit out a stack trace. Instead, `neptune-cli` should gracefully inform the user that a connection to `neptune-core` could not be made and that running `neptune-core` is a prerequisite.
