Return typed errors in object.Keystore.Decode func

And map them to http status codes in the
GET /object/path/.../kvstore/entry api handler.

	KeystoreErrNotExist => 404
	KeystoreErrKeyEmpty => 400

core/object/keystore_decode.go

@@ -11,10 +11,10 @@ func (t *keystore) decode(keyname string) ([]byte, error) {
 		err error
 	)
 	if keyname == "" {
-		return []byte{}, fmt.Errorf("key name can not be empty")
+		return []byte{}, KeystoreErrKeyEmpty
 	}
 	if !t.HasKey(keyname) {
-		return []byte{}, fmt.Errorf("key does not exist: %s", keyname)
+		return []byte{}, fmt.Errorf("%w: %s", KeystoreErrNotExist, keyname)
 	}
 	k := keyFromName(keyname)
 	if s, err = t.config.GetStrict(k); err != nil {

core/oxcmd/keystore_decode.go

@@ -66,6 +66,8 @@ func (t *CmdKeystoreDecode) RunForPath(ctx context.Context, c *client.T, path na
 		return fmt.Errorf("%s: %s", path, *response.JSON403)
 	case http.StatusInternalServerError:
 		return fmt.Errorf("%s: %s", path, *response.JSON500)
+	case http.StatusNotFound:
+		return fmt.Errorf("%s: %s", path, *response.JSON404)
 	default:
 		return fmt.Errorf("%s: unexpected response: %s", path, response.Status())
 	}

daemon/api/api.yaml

@@ -2400,6 +2400,8 @@ paths:
           $ref: '#/components/responses/401'
         403:
           $ref: '#/components/responses/403'
+        404:
+          $ref: '#/components/responses/403'
         500:
           $ref: '#/components/responses/500'