[#83] client: Implement status library

Define base `Status` interface. Provide the functionality to distinguish
success and failure returns. Provide functionality to transport statuses
over NeoFS API V2 protocol. Support success `OK` and failure `INTERNAL`
returns.

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

client/status/common.go

@@ -0,0 +1,55 @@
+package apistatus
+
+import (
+	"github.com/nspcc-dev/neofs-api-go/v2/status"
+)
+
+// ServerInternal describes failure statuses related to internal server errors.
+// Instances provide Status and StatusV2 interfaces.
+//
+// The status is purely informative, the client should not go into details of the error except for debugging needs.
+type ServerInternal struct {
+	v2 status.Status
+}
+
+func (x ServerInternal) Error() string {
+	return errMessageStatusV2(
+		globalizeCodeV2(status.Internal, status.GlobalizeCommonFail),
+		x.v2.Message(),
+	)
+}
+
+// implements method of the FromStatusV2 local interface.
+func (x *ServerInternal) fromStatusV2(st *status.Status) {
+	x.v2 = *st
+}
+
+// ToStatusV2 implements StatusV2 interface method.
+// If the value was returned by FromStatusV2, returns the source message.
+// Otherwise, returns message with
+//  * code: INTERNAL;
+//  * string message: empty;
+//  * details: empty.
+func (x ServerInternal) ToStatusV2() *status.Status {
+	x.v2.SetCode(globalizeCodeV2(status.Internal, status.GlobalizeCommonFail))
+	return &x.v2
+}
+
+// SetMessage sets message describing internal error.
+//
+// Message should be used for debug purposes only.
+func (x *ServerInternal) SetMessage(msg string) {
+	x.v2.SetMessage(msg)
+}
+
+// Message returns message describing internal server error.
+//
+// Message should be used for debug purposes only. By default, it is empty.
+func (x ServerInternal) Message() string {
+	return x.v2.Message()
+}
+
+// WriteInternalServerErr writes err message to ServerInternal instance.
+func WriteInternalServerErr(x *ServerInternal, err error) {
+	x.SetMessage(err.Error())
+}