I'm unsure I understand changing only the default __str__() method. You will anyway have to format the message differently depending on whether you raise from a from-import or a from-import-* or any other locations. 

AFAIU you "just" need the following 

  -        PyErr_SetImportError(PyUnicode_FromFormat("cannot import name %R", name), pkgname, pkgpath);
  +        PyErr_SetImportError(
  +                PyUnicode_FromFormat("cannot import name %R from %R (%S)",
  +                    name, pkgname, pkgpath),
  +                pkgname, pkgpath);

To use Barry format (though keeping quotes around identifiers to match current behavior).
(And similar if path is null). 

I'm unsure if you meant to provide a set of "format-template" to ImportError that are guarantied to be called with format(name=..., path=...) but I doubt it.