Changed error check on dispatch, so that errors in the callback code will
be properly signalled and not masked as OSC address errors.
See also https://groups.google.com/forum/#!topic/kivy-dev/GOkp9dWSeKw

dpj@cinnabon:~/data/repo/kivy.dpj.hg$ hg com
Processing '/etc/joe/jmacsrc'...Processing '/etc/joe/ftyperc'...done
done

Changed error check on dispatch, so that errors in the callback code will
be properly signalled and not masked as OSC address errors.
See also https://groups.google.com/forum/#!topic/kivy-dev/GOkp9dWSeKw
Submission #2, based on kivy repo in better state.