Mostly when I disconnect during upload with s3api clients I see the
proxy log a traceback for EPIPE, but if I set my my client_timeout low
and the proxy initiates the disconnect s3api will get surprised by the
499 response and return 500.
Now s3api will handle it the same as a RequestTimeout, which looks like
a 400 on the wire if anyone is still there.
Change-Id: I08be94fc5cf16679f41a2fd08ce1d52ce6300871