feat(tvix/eval): add error kind for unmergeable nested attributes

Change-Id: Ic5e6d1bf2625c33938360affb0d1a7c922af11bf
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6799
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/compiler/bindings.rs

@@ -48,7 +48,10 @@ impl Binding {
                 Some(ErrorKind::UnmergeableInherit { name: name.clone() })
             }
 
-            Binding::Plain { .. } => todo!(),
+            Binding::Plain { expr } => match expr {
+                ast::Expr::AttrSet(_) => todo!(),
+                _ => Some(ErrorKind::UnmergeableValue),
+            },
         }
     }
 }