Fixes #35 (handle missing Hanlon server properly)

The changes in this commit ensure that an understandable error is output if the CLI-based client is run while the Hanlon server is not running (or is inaccessible at the URL that the client is attempting to connect to based on its configuration). This commit also includes a small set of fixes to the config slice to avoid having `null` values included in the help messages returned by the that slice when these new errors are thrown.