Throwing a non-Error object in a script run by QJSEngine is not reported to the C++ API as an error

Suppose I have a `QJSEngine`. I run some JavaScript through this, and it throws something. This something is not an `Error`; it's anyone's guess as to what it actually is. It's reported as a plain old return value, and then I can't tell whether or not a weird exception was thrown.

This is a problem because I'm making a programming game where the user writes their own scripts; I can't guarantee that no one won't try to do a `throw 5;` to see what happens.