Handle case in which aiohttp closes the websocket due to lack of ping responses. (#453)
Handle case in which aiohttp closes the websocket due to lack of ping responses. (#453)

* Handle case in which aiohttp closes the websocket due to lack of ping responses.
https://github.com/slackapi/python-slackclient/issues/451

* Dispatch error event when error received over websocket

slackhq/python-slackclient,slackapi/python-slackclient,slackapi/python-slackclient,slackapi/python-slackclient