Die when key is not provided to recs-delta

usage() was not used correctly (oh the irony); it's a method not a
function and doesn't take an argument.  Additionally, not specifying
--key is a fatal usage error and exiting with error is more appropriate;
the top-level operation handler will catch the die and show the error
along with our usage doc.
Die when dbfile is not provided to recs-join

$this->usage(...) just returns a string; it doesn't print or die.  The
die() will be caught by the top-level operation handler and exit
appropriately with our usage.
Clean up argument checking style to read more naturally