code health: rename error iproto code

Since 2.4.1 IPROTO_ERROR (0x31) is renamed to IPROTO_ERROR_24 and
IPROTO_ERROR name is used for 0x52 constant describing extended error
info [1].

1. https://www.tarantool.io/en/doc/latest/dev_guide/internals/msgpack_extensions/#the-error-type

Part of #209

const.go

@@ -35,7 +35,7 @@ const (
 	KeyExpression   = 0x27
 	KeyDefTuple     = 0x28
 	KeyData         = 0x30
-	KeyError        = 0x31
+	KeyError24      = 0x31
 	KeyMetaData     = 0x32
 	KeyBindCount    = 0x34
 	KeySQLText      = 0x40

response.go

@@ -172,7 +172,7 @@ func (resp *Response) decodeBody() (err error) {
 				if resp.Data, ok = res.([]interface{}); !ok {
 					return fmt.Errorf("result is not array: %v", res)
 				}
-			case KeyError:
+			case KeyError24:
 				if resp.Error, err = d.DecodeString(); err != nil {
 					return err
 				}
@@ -262,7 +262,7 @@ func (resp *Response) decodeBodyTyped(res interface{}) (err error) {
 				if err = d.Decode(res); err != nil {
 					return err
 				}
-			case KeyError:
+			case KeyError24:
 				if resp.Error, err = d.DecodeString(); err != nil {
 					return err
 				}