uncaught error in eval frame re: assemblyscript_demo notebook

[This notebook](https://alpha.iodide.io/notebooks/1234/) demonstrates that, when you load the provided assemblyscript plugin, Iodide hangs.

In the devtools console, I see:

```
TypeError: can't redefine non-configurable property "signbit"
```

Regardless of why the error is being thrown (something to do with the plugin, in this case) we need to make sure to catch it in the eval.

/cc @sdwfrost (it's his notebook)