sl2: Improve prediction of screens.

This includes:

* When predicting, we do not stop on the first exception. Instead, we
  keep going, trying to predict what we can.

* This includes screen parameters and arguments as well.

* If we can't evaluate the list part of a for loop, we use [ 0 ] - in
  conjunction with the error handling, this should get us
  something. In many cases, it gets us everything important.

* Once per if statement, we predict all branches.