[#83] pkg/client: Support status returns

Make all `Client` methods to return structured values and error. Parse v2
status messages in all RPC and provide status getter from all result
structures. Returns status failures as status result instead of error.

Interface changes:
  * all methods return `<method>Res` structure;
  * rename some methods to be more clear;
  * unify TZ and SHA256 objecy payload hashing in single method.

Behavior changes:
  * client doesn't verify object header structure received via Object.Head.
    If the caller was tied to verification, now it must do it explicitly.

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

client/common.go

@@ -0,0 +1,80 @@
+package client
+
+import (
+	"fmt"
+
+	"github.com/nspcc-dev/neofs-api-go/v2/signature"
+	apistatus "github.com/nspcc-dev/neofs-sdk-go/client/status"
+)
+
+// common interface of resulting structures with API status.
+type resCommon interface {
+	setStatus(apistatus.Status)
+}
+
+// structure is embedded to all resulting types in order to inherit status-related methods.
+type statusRes struct {
+	st apistatus.Status
+}
+
+// setStatus implements resCommon interface method.
+func (x *statusRes) setStatus(st apistatus.Status) {
+	x.st = st
+}
+
+// Status returns server's status return.
+//
+// Use apistatus package functionality to handle the status.
+func (x statusRes) Status() apistatus.Status {
+	return x.st
+}
+
+// checks response signature and write client error if it is not correct (in this case returns true).
+func isInvalidSignatureV2(res *processResponseV2Res, resp responseV2) bool {
+	err := signature.VerifyServiceMessage(resp)
+
+	isErr := err != nil
+	if isErr {
+		res.cliErr = fmt.Errorf("invalid response signature: %w", err)
+	}
+
+	return isErr
+}
+
+type processResponseV2Prm struct {
+	callOpts *callOptions
+
+	resp responseV2
+}
+
+type processResponseV2Res struct {
+	statusRes resCommon
+
+	cliErr error
+}
+
+// performs common actions of response processing and writes any problem as a result status or client error
+// (in both cases returns true).
+//
+// Actions:
+//  * verify signature (internal);
+//  * call response callback (internal).
+func (c *clientImpl) processResponseV2(res *processResponseV2Res, prm processResponseV2Prm) bool {
+	// verify response structure
+	if isInvalidSignatureV2(res, prm.resp) {
+		return true
+	}
+
+	// handle response meta info
+	if err := c.handleResponseInfoV2(prm.callOpts, prm.resp); err != nil {
+		res.cliErr = err
+		return true
+	}
+
+	// set result status
+	st := apistatus.FromStatusV2(prm.resp.GetMetaHeader().GetStatus())
+
+	res.statusRes.setStatus(st)
+
+	return !apistatus.IsSuccessful(st)
+}