17.4.3 %ABORT Directive
  The %ABORT directive terminates the compilation and
  displays a message you provide.

  The text must be a quoted string literal. This information is
  displayed to SYS$ERROR and in the compilation listing if one
  is being created. BASIC stops the compilation and terminates
  the listing file as soon as it encounters a %ABORT directive,
  and so BASIC does not perform syntax checking on the re-
  mainder of the program. See Section 17.4.5 for an example of
  using %ABORT.