when compiling and using next-error, the error expression is not
correctly marked. Here is the patch from 
John Gerard Malecki <johnm@artisan.com> (against ocaml-mode 1.05, but it
can be applied trivially by hand):

635,637c635,636
<             (setq end (- (+ (point) end 1) beg) ;; (+ (point) end)
<                   beg (+ (point) 1) ;; (+ (point) beg)
<                   )
---
>             (setq beg (+ (point) beg)
>                   end (+ (point) end))