chore(tvix/eval): print value when panicking on internals

This makes it much easier to figure out what happened while debugging
this sort of thing.

Change-Id: I2e0e8096709adc647d63c04f213c547c415e5f44
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6418
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

tvix/eval/src/vm.rs

@@ -673,7 +673,9 @@ impl VM {
             | Value::AttrNotFound
             | Value::DynamicUpvalueMissing(_)
             | Value::Blueprint(_)
-            | Value::DeferredUpvalue(_) => panic!("tvix bug: internal value left on stack"),
+            | Value::DeferredUpvalue(_) => {
+                panic!("tvix bug: internal value left on stack: {:?}", value)
+            }
 
             _ => Ok(()),
         }