network uploads: QNonContiguousByteDevice does not report error

If the underlying QIODevice returns -1 (e.g. device has not been opened for reading or so), the QNonContiguousByteDevice returns 0, which could also mean we are waiting for more data.
This means the HTTP and SPDY upload handlers cannot decide whether there was an error or not while uploading.

We should either return -1 as well, or have some other means of reporting errors.