While investigating another problem (which turned out to be not the fault of KDE at all), I encountered this mysterious message on stderr:

Syntax Error: Not enough arguments

It took a long time to track down the source of this message and find out that it was from kioexec.  If kioexec had given its name in the error message, as most Unix commands do, then the source of the message would have been obvious and made investigation easier.

Built kioclient with these changes, verified error message output.

$ kioclient exec
kioclient: Syntax error, not enough arguments
$ kioclient exec a b c
kioclient: Syntax error, too many arguments
$ kioclient foo
kioclient: Syntax error, unknown command 'foo'
$