Update Request#parseNetworkResponse javadoc to disallow returning null

[jpd236]

Request#parseNetworkResponse claims it's okay to return null if a response can't be parsed. In reality, it seems like this would almost guarantee an NPE either in CacheDispatcher or ExecutorDelivery, so unless someone has an entirely custom ResponseDelivery, they probably already are returning Response.error rather than null. We should verify this and consider updating the Javadoc accordingly.

[uhager]

To add to this, if someone uses a ResponseDelivery that does not call ExecutorDelivery, they also have to implement their own RequestQueue, since both Request#finish and RequestQueue#finish are package-private, and are the only way of removing the request from the RequestQueue's current requests.