[#140] apistatus: Support ACCESS_DENIED error

Define `ObjectAccessDenied` type for `ACCESS_DENIED` code. Provide
method to write/read human-readable reason.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

client/status/object_test.go

@@ -0,0 +1,26 @@
+package apistatus_test
+
+import (
+	"testing"
+
+	apistatus "github.com/nspcc-dev/neofs-sdk-go/client/status"
+	"github.com/stretchr/testify/require"
+)
+
+func TestObjectAccessDenied_WriteReason(t *testing.T) {
+	const reason = "any reason"
+
+	var st apistatus.ObjectAccessDenied
+
+	res := st.Reason()
+	require.Empty(t, res)
+	detailNum := apistatus.ToStatusV2(st).NumberOfDetails()
+	require.Zero(t, detailNum)
+
+	st.WriteReason(reason)
+
+	res = st.Reason()
+	require.Equal(t, reason, res)
+	detailNum = apistatus.ToStatusV2(st).NumberOfDetails()
+	require.EqualValues(t, 1, detailNum)
+}