[echo_client] Exit when an extension is enabled and an old protocol which does not support extensions is specified
[echo_client] Exit when an extension is enabled and an old protocol which does not support extensions is specified

Release note: Exit echo_client when an extension is enabled and an old protocol is specified.

Review URL: https://codereview.appspot.com/9072043
GoogleChromeLabs/pywebsocket3,googlearchive/pywebsocket,googlearchive/pywebsocket,GoogleChromeLabs/pywebsocket3,google/pywebsocket,google/pywebsocket,googlearchive/pywebsocket,GoogleChromeLabs/pywebsocket3,google/pywebsocket