ref/tools/chopmod [TASK]: print meaningful error message

src/synt/ref/tools/chopmod.f

@@ -399,7 +399,7 @@ c open output
       if (replace) then
         open(lu, file=outfile, err=99)
       else
-        open(lu, file=outfile, status='new', err=99)
+        open(lu, file=outfile, err=96, status='new')
       endif
       write(lu, '(a72)', err=98) text
 c----------------------------------------------------------------------
@@ -475,6 +475,8 @@ c   53 format(18x,a)
    99 stop 'ERROR: opening output file'
    98 stop 'ERROR: writing output file'
    97 stop 'ERROR: closing output file'
+   96 print *,'ERROR: opening output file'
+      stop 'ERROR: if output file exists, consider option -o'
       end
 c======================================================================
 c subroutines