GUILE debug: Revert part of 52bea08ef73a55ee, so the file of an error is shown

GUILE debug: Revert part of 52bea08ef73a55ee, so the file of an error is shown

When debug is set to #f, a guile error, like for example (cdr '()) will only
show the error message itself, but no indication whatsoever where the error
occured. This makes debugging scheme bugs in our lilypond code basically
impossible.

This patch properly fixes the debug settings, by either setting 'debug for
guile <2.0 or setting the finer-grained debug options for guile >=2.0.

That's also the correct fix that Ian Hulin suggest in a private mail.

Not tested with guile 2.0, though.