[#140] client: Specify status errors of Client methods

Extend docs with supported status returns. Add several helper functions
which allow to check the particular status.

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

client/errors.go

@@ -0,0 +1,42 @@
+package client
+
+import apistatus "github.com/nspcc-dev/neofs-sdk-go/client/status"
+
+// IsErrContainerNotFound checks if err corresponds to NeoFS status
+// return corresponding to missing container.
+func IsErrContainerNotFound(err error) bool {
+	switch err.(type) {
+	default:
+		return false
+	case
+		apistatus.ContainerNotFound,
+		*apistatus.ContainerNotFound:
+		return true
+	}
+}
+
+// IsErrObjectNotFound checks if err corresponds to NeoFS status
+// return corresponding to missing object.
+func IsErrObjectNotFound(err error) bool {
+	switch err.(type) {
+	default:
+		return false
+	case
+		apistatus.ObjectNotFound,
+		*apistatus.ObjectNotFound:
+		return true
+	}
+}
+
+// IsErrObjectAlreadyRemoved checks if err corresponds to NeoFS status
+// return corresponding to already removed object.
+func IsErrObjectAlreadyRemoved(err error) bool {
+	switch err.(type) {
+	default:
+		return false
+	case
+		apistatus.ObjectAlreadyRemoved,
+		*apistatus.ObjectAlreadyRemoved:
+		return true
+	}
+}