flux-module: improve error message / require user input

[chu11]

> flux module stats
flux-module: broker.stats-get: Function not implemented

A cursory scan did not find an oldbroker.stats-get, so I bet this was set as the default because "broker.FOO" was the most common default thing to do a long time ago.

Perhaps should just make sure user supplied a module name and/or give a much better error message.