If r.Context() is already canceled when the cache attempt returns no
chunks, the cache failure is almost certainly a side-effect of the
client disconnecting, not real backpressure. Surface the context error
so GetObjectHandler logs at V(3) and skips writing a response, instead
of synthesizing a 503 that nobody will read.
Addresses Gemini review feedback on #9233.
Chris Lu
18b51253da