Add instructions for error handling resuming

[zippy]

when an error is caught by the error handler, it has to be able to restart the runtree at various points, i.e. from the root (which is like a "panic & return") or from the same point as the function that thew the error (with possibly some other value) or from some parent (or child) with a different value which acts as a RETRY.

[zippy]

ca6be1f adds CONTINUE instruction, which acts like a kind of return, or goto, which allows you to restart reduction at any point in the tree

We still need to add error checking of some sort so that you can really only continue from valid places, i.e. a parent, and maybe a next child or something. Also if continuing from a flow-control spot, there may be some special cases to clean up.