Every 1000-2000 Calls Yields a Malformed Response

Error thrown: 
```
TypeError: Cannot read property 'response' of undefined
    at Request._callback (T260Gtumblr/node_modules/tumblr.js/lib/tumblr.js:243:31)
    at Request.self.callback (T260Gtumblr/node_modules/tumblr.js/node_modules/request/main.js:122:22)
    at Request.EventEmitter.emit (events.js:98:17)
    at Request.<anonymous> (T260Gtumblr/node_modules/tumblr.js/node_modules/request/main.js:661:16)
    at Request.EventEmitter.emit (events.js:117:20)
    at IncomingMessage.<anonymous> (T260Gtumblr/node_modules/tumblr.js/node_modules/request/main.js:623:14)
    at IncomingMessage.EventEmitter.emit (events.js:117:20)
    at _stream_readable.js:910:16
    at process._tickCallback (node.js:415:13)
```

This patch ensures the body has a `response`, and calls back with:
```JavaScript
new Error('API error: malformed API response (no response on line)')
```
Rather than crashing the instance.