Patch to add a debugging string to HExtlib.split_nth reverted

Patch to add a debugging string to HExtlib.split_nth reverted
(we cannot add a string to every function that raises an exception).

To know where the problem is, it is sufficient to look at the backtrace.
However:
 1) it works for matitac -debug
 2) ocaml is so dumb that it says "compile with -g" when it cannot find
    the executable in the current working directory. Since Matita chdir
    to the directory where the .ma file is, you need to put a copy of
    matitac (or make a symbolic link) where the .ma file is.