lib: Fix silent assertitons

There are a couple of assert statements in the codebase which can make jw-pkg fail without any detail whatsoever if --backtrace is not specified, fix that.
Signed-off-by: Jan Lindemann <jan@janware.com>

src/python/jw/pkg/lib/Cmd.py

@@ -48,7 +48,7 @@ class Cmd(abc.ABC): # export
                 if isinstance(parent, App):
                     self.__app = parent
                     break
-                assert parent != parent.__parent
+                assert parent != parent.__parent, f'Assertion failed: Parent mismatch'
                 parent = parent.__parent
         return self.__app