TwythonStreamer on_error in examples and documentation is wrong

[EhsanKia]

This pull request added headers to the on_error call: https://github.com/ryanmcgrath/twython/pull/504

The issue is that all other the examples and documentation show on_error as taking two parameters, so they are all broken. I copied the code from here: https://twython.readthedocs.io/en/latest/usage/streaming_api.html

Looking at the codebase, here are the places that need to be updated: https://github.com/ryanmcgrath/twython/blob/master/docs/usage/streaming_api.rst https://github.com/ryanmcgrath/twython/blob/master/examples/stream.py https://github.com/ryanmcgrath/twython/blob/master/tests/test_streaming.py

I can try submitting a PR requests, though updating those seems fairly trivial.