Upstream: no last buffer on errors. Previously, after sending a header we always sent a last buffer and finalized a request with code 0, even in case of errors. In some cases this resulted in a loss of ability to detect the response wasn't complete (e.g. if Content-Length was removed from a response by gzip filter). This change tries to propogate to a client information that a response isn't complete in such cases. In particular, with this change we no longer pretend a returned response is complete if we wasn't able to create a temporary file. If an error code suggests the error wasn't fatal, we flush buffered data and disable keepalive, then finalize request normally. This allows to to propogate information about a problem to a client, while still sending all the data we've got from an upstream.