to be merged: log kms error from json message

Author: urykhy

kms.go

@@ -160,7 +160,20 @@ func (c *Client) kmsDecrypt(url string, requestBody []byte) ([]byte, error) {
 	}
 
 	if statusCode != 200 {
-		return nil, fmt.Errorf("unexpected response code from KMS: %v", statusCode)
+		// On error, kms respond with error message in JSON object.
+		type Exception struct {
+			RemoteException struct {
+				Message string `json:"message"`
+			}
+		}
+		var kmsException Exception
+		if err = json.Unmarshal(responseBody, &kmsException); err == nil && kmsException.RemoteException.Message != "" {
+			errorMessage := kmsException.RemoteException.Message
+			err = fmt.Errorf("unexpected response code from KMS: %v: %v", statusCode, errorMessage)
+		} else {
+			err = fmt.Errorf("unexpected response code from KMS: %v", statusCode)
+		}
+		return nil, err
 	}
 
 	type KmsRespose struct {