handle error cases more gracefully in v3

[hjdivad]

When something downstream adds nodes and then errors the stack can get imbalanced.

This is less of a problem in v0.3, but there's still a couple of things we likely want to do

1. indicate that things are unbalanced in the output
2. autoclose for .stack &c.

[hjdivad]

For both v0.2 and v0.3, I'm tempted to make stop unwind the stack (ie keep stopping until the token's node is stopped). This makes it easier to do things like instrument builds in ember-cli and detach a build subtree after reporting it, even if downstream has created something unbalanced.

@stefanpenner @runspired @rwjblue what do you think?

[hjdivad]

we should mark autoclosed nodes so analysis can highlight that the subgraph stats totals are correct, but within the individual subgraph the stats might be misallocated

[runspired]

this should already happen in v3, or perhaps the work I did with @chadhietala didn't get pushed up?

[hjdivad]

@runspired cool we just need to work it in to the rest of the ecosystem (eg heimdalljs-graph)