Return unknown option name as error

embddr/optargs.scm

@@ -58,7 +58,7 @@
         opt-spec)
    ;; unrecognized options
    (lambda (opt name arg opts rest error)
-     (values opts rest #t))
+     (values opts rest name))
    ;; operands
    (lambda (operand opts rest error)
      (values opts (cons operand rest) error))