allow subclasses of HttpConnection to read error

I'm working on integrating Raven into my App Engine application. 

The problem with this is that since the platform does not support threads I need to split the whole process into 2 parts: (1) collect and marshal the event & (2) send it. The last part is being done with an asynchronous message queue.

So I subclassed `HttpConnection` in order to re-use most of the logic for sending the event. But my implementation receives a `String` and not an `Event`. So I wrote my own `doSend` method, but obviously want to re-use the `getErrorMessageFromStream` - hence this change.