Updated error page template

I removed some unused imports and changed "font-family: sans" to "font-family: sans-serif", which I assume it was meant to be.
Merge pull request #597 from a-e-m/master

Updated error page template
Fix sample websocket via gevent-websocket
update api: https://pypi.python.org/pypi/gevent-websocket/
Merge pull request #598 from avelino/doc-async-websocket-sample

Fix sample websocket via gevent-websocket