make error message more usefuly by revealing type

gitlab.go

@@ -19,12 +19,12 @@ package gitlab
 import (
 	"bytes"
 	"encoding/json"
-	"errors"
 	"fmt"
 	"io"
 	"io/ioutil"
 	"net/http"
 	"net/url"
+	"reflect"
 	"strconv"
 	"strings"
 
@@ -348,7 +348,7 @@ func parseID(id interface{}) (string, error) {
 	case string:
 		return v, nil
 	default:
-		return "", errors.New("the ID must be an int or a string")
+		return "", fmt.Errorf("the ID must be an int or a string but is %v", reflect.TypeOf(id))
 	}
 }