message: Clarify error messages when CompletionQueue returns not ok.
diff: @@ -123,7 +123,9 @@ class RPCState : public GrpcClientCQTag {
  if (s.ok() && !ok) {
  // Since this function is only being used for processing the response
  // to Finish for client-side unary calls, ok should never be false
  - s.Update(errors::Internal("unexpected ok value at rpc completion"));
  + s.Update(
  + errors::Internal("GRPC status is okay but CompletionQueueStatus is "
  + "not. This should never happen."));
  }
  if (s.ok()) {
  @@ -453,8 +455,8 @@ class StreamingRPCState : public UntypedStreamingRPCState {
  if (!ok) {
  // unlocks mu_
  MarkDoneAndCompleteExchanges(errors::Internal(
  - "Unexpected ok value at streaming rpc writing. ",
  - "Probably because the completion queue has been shut ",
  + "Not ok value returned by CompletionQueue when attempting streaming "
  + "rpc write. Probably because the completion queue has been shut "
  "down or the connection went down. ",
  context_->debug_error_string()));
  return;
  @@ -511,7 +513,8 @@ class StreamingRPCState : public UntypedStreamingRPCState {
  Status s = FromGrpcStatus(call_status_);
  if (s.ok() && !ok) {
  s.Update(
  - errors::Internal("unexpected ok value at streaming rpc completion. ",
  + errors::Internal("GRPC status is okay but CompletionQueueStatus is "
  + "not. This should never happen.",
  context_->debug_error_string()));
  }
  // unlocks mu_
  