fix crash on faulty API response

To prevent this:

    mike@vps674:~/nodeprojects/gekko/c$ cat mtgox.err 

    /home/mike/nodeprojects/gekko/node_modules/mtgox-apiv2/mtgox.js:50
              callback(new Error("Request failed with " + res.statusCode));
                                                             ^
    TypeError: Cannot read property 'statusCode' of undefined
        at Request._callback (/home/mike/nodeprojects/gekko/node_modules/mtgox-apiv2/mtgox.js:50:58)
        at Request.init.self.callback (/home/mike/nodeprojects/gekko/node_modules/mtgox-apiv2/node_modules/request/index.js:148:22)
        at Request.EventEmitter.emit (events.js:88:17)
        at ClientRequest.Request.init.self.clientErrorHandler (/home/mike/nodeprojects/gekko/node_modules/mtgox-apiv2/node_modules/request/index.js:257:10)
        at ClientRequest.EventEmitter.emit (events.js:88:17)
        at CleartextStream.socketErrorListener (http.js:1330:9)
        at CleartextStream.EventEmitter.emit (events.js:88:17)
        at SecurePair.exports.connect.cleartext._controlReleased (tls.js:1267:15)
        at SecurePair.EventEmitter.emit (events.js:88:17)
        at SecurePair.error (tls.js:921:10)