NameError may either include offending name or not. Unfortunately, this
change makes test float-dependent. And using integer division leads to
different error message than CPython.
This makes exception traceback info self contained (ie doesn't rely on
list object, which was a bit of a hack), reduces code size, and reduces
RAM footprint of exception by eliminating the list object.
Addresses part of issue #1126.
Issue was with uPy: on local machine with micropython-lib installed, io
module is available. Not the case on Travis CI, where only _io module
is available in uPy.
