Calling scylla-nodetool with option describering and ommiting the keyspace
name argument results in a boost exception with the following error message:
error running operation: boost::wrapexcept<boost::bad_any_cast> (boost::bad_any_cast: failed conversion using boost::any_cast)
This change checks for the missing keyspace and outputs a more sensible
error message:
error processing arguments: keyspace must be specified
Closesscylladb/scylladb#17741