tolerate unbalanced stop

[hjdivad]

These are often the result of errors that we are masking and whose instrumentation information is unimportant.

Similarly we should make it easier to disable instrumentation so that if users run into problems (eg because of a bad broccoli plugin that has unbalanced nodes) they can work around it.

This also needs to not be a problem in v0.3 although I'm fairly sure i've talked with @runspired about this and it's not; although we should make sure there's a test for it