Report an error instead of crashing when iteration encounters non-music.

Report an error instead of crashing when iteration encounters non-music.

The parser checks that it got music where expected, but Scheme functions
 like make-music don't, so it's easy to create invalid music. Previously
this often crashed Lilypond.

There are some existing checks that silently ignore non-music instead of
reporting an error. Probably most of these should be changed.

[dak, 2014-02-14 10:54:12]

Shouldn't most of those get caught earlier by making use of the types defined in
scm/define-music-properties.scm ?

If we are missing proper typechecks in make-music et al, maybe we should fix
that there?

[Devon Schudy, 2014-02-16 02:56:14]

dak wrote:
> Shouldn't most of those get caught earlier by making use of the types
> defined in scm/define-music-properties.scm ?
>
> If we are missing proper typechecks in make-music et al, maybe we should
> fix that there?

They're already caught in make-music by -dcheck-internal-types, but
that's disabled by default (presumably for performance, although it
only slows make check by less than 1%).