Provide schema information on error

[johntrimble]

When config builder errors out, we get errors like the following:

Error processing request: throw+: {:type :InvalidConfigKeys, :message "Invalid config keys for DSE version 4.0.0: jvm-server"}

These errors are not exactly the most helpful error messages imaginable. When invalid input is provided, we should give some feedback as to what a valid option might be. In this case, only so many values are possible at this level of the schema, so we could just list them. If we did that, one would realize they should have used "jvm-server-options" instead of "jvm-server". Without that, it's a frakking mystery why it's not working.