fix(zentao): update error message when testing connection fails (#7175) (#7176)

Co-authored-by: Lynwee <linwei.hou@merico.dev>
Co-authored-by: houlinwei <timeror@gmail.com>
diff --git a/backend/plugins/zentao/models/access_token.go b/backend/plugins/zentao/models/access_token.go
index 60bf261..d084348 100644
--- a/backend/plugins/zentao/models/access_token.go
+++ b/backend/plugins/zentao/models/access_token.go
@@ -24,4 +24,5 @@
 
 type ApiAccessTokenResponse struct {
 	Token string `json:"token"`
+	Error string `json:"error"`
 }
diff --git a/backend/plugins/zentao/models/connection.go b/backend/plugins/zentao/models/connection.go
index ccce201..0554450 100644
--- a/backend/plugins/zentao/models/connection.go
+++ b/backend/plugins/zentao/models/connection.go
@@ -50,7 +50,11 @@
 		return errors.HttpStatus(http.StatusBadRequest).Wrap(err, "failed UnmarshalResponse for tokenResBody")
 	}
 	if tokenResBody.Token == "" {
-		return errors.HttpStatus(http.StatusBadRequest).New("failed to request access token")
+		msg := "failed to request access token"
+		if tokenResBody.Error != "" {
+			msg = tokenResBody.Error
+		}
+		return errors.HttpStatus(http.StatusBadRequest).New(msg)
 	}
 	apiClient.SetHeaders(map[string]string{
 		"Token": fmt.Sprintf("%v", tokenResBody.Token),