Don't stringify variables that aren't suitable.

Related to #38 though potentially not fixed.

scone/head/dependency_tracking.py

@@ -215,7 +215,9 @@ class DependencyVarProxy:
             self._tracker.register_variable(self._current_path_prefix, raw_value)
             return str(raw_value)
         else:
-            return repr(self)
+            raise ValueError(
+                f"Trying to stringify a variable proxy which contains {repr(self)}"
+            )
 
     def raw_(self) -> Union[Dict[str, Any], List[Any], int, str, bool]:
         if not self._current_path_prefix: