Improve error message if assertion fails.

Cython/Compiler/ExprNodes.py

@@ -13023,7 +13023,7 @@ class PyTypeTestNode(CoercionNode):
         #  The arg is known to be a Python object, and
         #  the dst_type is known to be an extension type.
         assert dst_type.is_extension_type or dst_type.is_builtin_type, \
-            "PyTypeTest on non extension type %s" % dst_type
+            "PyTypeTest for %s against non extension type %s" % (arg.type, dst_type)
         CoercionNode.__init__(self, arg)
         self.type = dst_type
         self.result_ctype = arg.ctype()