handle failure in ODE solving

[soonhokong]

@scungao wrote at https://github.com/dreal/dreal3/issues/64#issuecomment-95190217:

When it diverges, the ODE solver does not provide a "well-defined" pruning operator, which makes the procedure not delta-complete. I think we should return "ODE pruning fails" in those cases instead of delta-sat.