[fine] Many test improvements, error improvements

- Check for more error conditions
- Move to more definitive error assertions
- Simpler error messages in some cases
- Test more conditions more thoroughly, revisit old tests

fine/src/vm.rs

@@ -13,7 +13,8 @@ pub enum VMErrorCode {
     #[error("internal error: stack type mismatch: {0:?} is not {1:?}")]
     StackTypeMismatch(StackValue, Type),
 
-    // TODO: This one is *not* like the others!
+    // TODO: This one is *not* like the others! Distinguish between internal
+    //       errors and user errors?
     #[error("divide by zero")]
     DivideByZero,