[#520] objectcore: Refactor format validator

Remove redundant FIXME.
Move error to consts.

Signed-off-by: Dmitrii Stepanov <d.stepanov@yadro.com>

pkg/core/object/fmt.go

@@ -64,6 +64,8 @@ var errNoExpirationEpoch = errors.New("missing expiration epoch attribute")
 
 var errTombstoneExpiration = errors.New("tombstone body and header contain different expiration values")
 
+var errMissingSignature = errors.New("missing signature")
+
 func defaultCfg() *cfg {
 	return new(cfg)
 }
@@ -135,8 +137,7 @@ func (v *FormatValidator) Validate(ctx context.Context, obj *objectSDK.Object, u
 func (v *FormatValidator) validateSignatureKey(obj *objectSDK.Object) error {
 	sig := obj.Signature()
 	if sig == nil {
-		// TODO(@cthulhu-rider): #468 use "const" error
-		return errors.New("missing signature")
+		return errMissingSignature
 	}
 
 	var sigV2 refs.Signature
@@ -157,8 +158,6 @@ func (v *FormatValidator) validateSignatureKey(obj *objectSDK.Object) error {
 		return v.checkOwnerKey(*obj.OwnerID(), key)
 	}
 
-	// FIXME: #1159 perform token verification
-
 	return nil
 }