Show: add argument to NotAContainerError class constructor

The class `NotAContainerError` is used as an exception.
Where it is used it is initialized with an argument,
but the definition didn't accept one; now we accept an argument
in the constructor.

src/Mod/Show/Containers.py

@@ -255,7 +255,7 @@ def VisGroupChain(feat):
 class ContainerError(RuntimeError):
     pass
 class NotAContainerError(ContainerError):
-    def __init__(self):
-        ContainerError.__init__(self, u"{obj} is not recognized as container".format(obj.Name))
+    def __init__(self, name="None"):
+        ContainerError.__init__(self, "'{}' is not recognized as container".format(name))
 class ContainerTreeError(ContainerError):
     pass