addrConn: Report underlying connection error in RPC error #1855
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This only happens when the RPC failed because all addrConn was in TransientFailure. And this is best effort because the returned error might not be the real cause.
But it should be good enough to detect the reason when the ClientConn is "dead" (keep retrying but none of the connection works).
This is not a perfect solution because it's racy. But this should be a good workaround for most cases.
An ultimate solution would be to pass the errors as part of the connectivity state information to the balancers, and balancers pass those errors back to RPCs as a pick result. This would need API changes on balancers.
updates #1742
fixes #1633
fixes #1656