Have the type system track whether a given expression could error

[kozross]

In theory, this is possible to do: we know which builtins are capable of erroring, and can propagate this upwards. This is to some degree an overapproximation, but it might be useful.

This is dependent on #8 and #9 , assuming we want to do it at all. Potentially though, this could be done with #6 .