simplify unmarshal (#191)

* simplify unmarshal

* simplify unmarshalError

* rename errorAccumulate -> defaultErrorAccumulator

* update converage

error_accumulator.go

@@ -8,7 +8,7 @@ import (
 
 type errorAccumulator interface {
 	write(p []byte) error
-	unmarshalError() (*ErrorResponse, error)
+	unmarshalError() *ErrorResponse
 }
 
 type errorBuffer interface {
@@ -17,19 +17,19 @@ type errorBuffer interface {
 	Bytes() []byte
 }
 
-type errorAccumulate struct {
+type defaultErrorAccumulator struct {
 	buffer      errorBuffer
 	unmarshaler unmarshaler
 }
 
 func newErrorAccumulator() errorAccumulator {
-	return &errorAccumulate{
+	return &defaultErrorAccumulator{
 		buffer:      &bytes.Buffer{},
 		unmarshaler: &jsonUnmarshaler{},
 	}
 }
 
-func (e *errorAccumulate) write(p []byte) error {
+func (e *defaultErrorAccumulator) write(p []byte) error {
 	_, err := e.buffer.Write(p)
 	if err != nil {
 		return fmt.Errorf("error accumulator write error, %w", err)
@@ -37,15 +37,15 @@ func (e *errorAccumulate) write(p []byte) error {
 	return nil
 }
 
-func (e *errorAccumulate) unmarshalError() (*ErrorResponse, error) {
-	var err error
-	if e.buffer.Len() > 0 {
-		var errRes ErrorResponse
-		err = e.unmarshaler.unmarshal(e.buffer.Bytes(), &errRes)
-		if err != nil {
-			return nil, err
-		}
-		return &errRes, nil
+func (e *defaultErrorAccumulator) unmarshalError() (errResp *ErrorResponse) {
+	if e.buffer.Len() == 0 {
+		return
 	}
-	return nil, err
+
+	err := e.unmarshaler.unmarshal(e.buffer.Bytes(), &errResp)
+	if err != nil {
+		errResp = nil
+	}
+
+	return
 }