Improve logging on web socket connection closed

[nirben82]

Describe the bug
When using web sockets and a connection is closed, an error is logged which can be confusing -

mesop/mesop/server/server.py

Line 355 in be13918

logging.error("WebSocket error: %s", e)

Expected behavior
When simple-socket closes the connection, it raises a ConnectionClosed exception, which does not indicate an error. I think it would be useful to catch this specific exception and log at info level that the connection was closed and the reason code.