Errors: context cancelled or context deadline exceeded are exposed as codes.Canceled, codes.DeadlineExceeded instead of 'codes.Unknown'

[ptabor]

The reason for this PR are test flakes like: https://travis-ci.com/github/etcd-io/etcd/jobs/500367259

    logger.go:130: 2021-04-22T07:56:30.190Z	WARN	m0	apply request took too long	{"member": "m0", "took": "898.215659ms", "expected-duration": "100ms", "prefix": "read-only range ", "request": "key:\"a\" ", "response": "", "error": "context deadline exceeded"}
    logger.go:130: 2021-04-22T07:56:30.190Z	INFO	m0	trace[566305530] range	{"member": "m0", "detail": "{range_begin:a; range_end:; }", "duration": "898.32003ms", "start": "2021-04-22T07:56:29.292Z", "end": "2021-04-22T07:56:30.190Z", "steps": ["trace[566305530] 'agreement among raft nodes before linearized reading'  (duration: 898.212117ms)"], "step_count": 1}
    logger.go:130: 2021-04-22T07:56:30.190Z	WARN	m0	request stats	{"member": "m0", "start time": "2021-04-22T07:56:29.292Z", "time spent": "898.37779ms", "remote": "@", "response type": "/etcdserverpb.KV/Range", "request count": 0, "request size": 3, "response count": 0, "response size": 0, "request content": "key:\"a\" "}
    network_partition_test.go:140: Op returned error: rpc error: code = Unknown desc = context deadline exceeded
    network_partition_test.go:141: Cancelling...
    network_partition_test.go:147: #0: expected 'expected error', got 'rpc error: code = Unknown desc = context deadline exceeded'