Ignore too many tones when playing back DTMF tones

darcs-hash:20090514110709-58103-8b115eae42c7bac558c7127118dc5bab5e33036f
Ignore error in tone playback in sipsimple.util.PersistentTones

darcs-hash:20090514110805-58103-d7c5e4a56299eedd4eee0907068003349960555a