iostream.py: Catches AttributeError on do_handshake().

Re: chfoo/wpull#98
iostream.py: Reraises AttributeError as NetworkError.
app.py: Removes blocking ioloop tracebacks.

Closes chfoo/wpull#97.
Merge branch 'issue/98-attributeerror' into develop
changelog: Updates entry for issue 98.

Closes chfoo/wpull#98