Engine error handling

[funwithtriangles]

We're now catching errors when sketches are first loaded by the engine, and just do an console.error. Would be good if there was some proper way for the engine to pass those errors on to whatever is consuming the engine (e.g. @hedron/desktop). Two options:

• Have an errors part of engine state, which is updated whenever an error is caught
• Have an onError method on the main engine class

I'm leaning towards just having a method, it's simpler and less opinionated