Return error when the protocol layer detects a framing error. More data
was supposed to be delivered than what actually did arrive.
Error code returned for this: NS_ERROR_NET_PARTIAL_TRANSFER
In HTTP1.1 for Content-Length: and chunked-encoding underruns
In http2 and SPDY for framing errors when data has already been received.
imgRequest::OnStopRequest will keep partially loaded images shown but
remove them from cache.
