Avoid Failure at all costs

[charlesetc]

We use Failure to match against splice — instead move to a custom defined exception

[cyrus-]

Is this still open?

[charlesetc]

I'm not sure sure how we do it, but we do run the challenge that if something shadows the Failure constructor relit stops working, so... we should probably find a way.

[cyrus-]

should be able to locally define a different exception pretty easily I think?