automatic error checking doesn't work with multiple contexts

[3b]

error checking code closes over a flag used to track when it is in a begin/end pair, but it should be storing the flag per context...

[3b]

Actually looks like it uses a special instead of a closer now, so it can work with multiple contexts with changes to user code. Probably needs better documentation though, and probably should be hidden behind some more generic API that could be extended to handle extension pointers etc. like with-context or whatever