Update sockjs-protocol-dev.py

According to HTTP/1.1 the status code 500 means "Internal Server Error". It would make much more sense to use 400 instead, meaning "Bad Request" in case of invalid json, empty json or any other invalid request.
fixed typo in error code (4500->400)
Merge pull request #62 from igm/patch-3

Use 400 HTTP response instead of 500 for invalid requests