adding name of duplicate name to error message (#1893)

* adding name of duplicate name to error message

* Black fix

---------

Co-authored-by: AU <adam-urbanczyk@users.noreply.github.com>

Author: shimwell

cadquery/assembly.py

@@ -218,7 +218,9 @@ def add(self, arg, **kwargs):
             # enforce unique names
             name = kwargs["name"] if kwargs.get("name") else arg.name
             if name in self.objects:
-                raise ValueError("Unique name is required")
+                raise ValueError(
+                    f"Unique name is required. {name} is already in the assembly"
+                )
 
             subassy = arg._copy()