Handle invalid input/json gracefully

```
[2013-03-18 22:17:20.754] [DEBUG] [default] - error     Exception on "POST /884/3xlw93mk/xhr_send" in filter "xhr_send":
SyntaxError: Unexpected token )
    at Object.parse (native)
    at SockJSConnection.module.exports.sockjsServer.on.connection.on._.each.channel (/usr/lib/node_modules/socket-redis/lib/worker.js:127:17)
    at SockJSConnection.EventEmitter.emit (events.js:93:17)
    at Session.didMessage (/usr/lib/node_modules/socket-redis/node_modules/sockjs/lib/transport.js:212:25)
    at App.exports.app.xhr_send (/usr/lib/node_modules/socket-redis/node_modules/sockjs/lib/trans-xhr.js:81:15)
    at execute_request (/usr/lib/node_modules/socket-redis/node_modules/sockjs/lib/webjs.js:21:38)
    at exports.generateHandler.req.next_filter (/usr/lib/node_modules/socket-redis/node_modules/sockjs/lib/webjs.js:95:18)
    at IncomingMessage.exports.GenericApp.GenericApp.expect_xhr.status (/usr/lib/node_modules/socket-redis/node_modules/sockjs/lib/webjs.js:272:16)
    at IncomingMessage.EventEmitter.emit (events.js:90:17)
    at IncomingMessage._emitEnd (http.js:366:10)
```