Fix #743: reinstate warning for unterminated span dynamics.

[reinhold . kainhofer]

I can't really judge the internals of the patch. But at least I don't
see any obvious problem.
On the other hand, I would really like this to be included, as that
issue has bitten me quite a few times while preparing ~50 small excerpts
of Dvorak's Cello concerto for a thesis...

http://codereview.appspot.com/33055