Issue with `_sesame_plan_with_fast_downward`

[yichao-liang]

With the current code in _sesame_plan_with_fast_downward, bilevel planning will halt with an exception if the first task plan found is not refinable.

An easy fix would it make it to raise an _DiscoveredFailureException but wasn't sure it's the best choice.

[NishanthJKumar]

Thanks for opening an issue! The fix is not straightforward, because raising the exception will not make FD produce a different plan. This is a deeper issue /feature request that has been on our backlog for a while; but there's no straightforward/clear way to fix unfortunately