fix(tvix/eval): correct runtime error for missing dynamic upvalue

Change-Id: I75c351619780fdc5186a54f3df9b244ada984069
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6324
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/vm.rs

@@ -381,6 +381,12 @@ impl VM {
 
                 OpCode::OpGetUpvalue(upv_idx) => {
                     let value = self.frame().closure.upvalue(upv_idx).clone();
+                    if let Value::DynamicUpvalueMissing(name) = value {
+                        return Err(
+                            ErrorKind::UnknownDynamicVariable(name.as_str().to_string()).into()
+                        );
+                    }
+
                     self.push(value);
                 }