[MM-45991] Check and return JSON errors (#20735)

app/import_validators.go

@@ -332,10 +332,10 @@ func validateUserTeamsImportData(data *[]UserTeamImportData) *model.AppError {
 			}
 		}
 
-		if tdata.Theme != nil && 0 < len(strings.Trim(*tdata.Theme, " \t\r")) {
+		if tdata.Theme != nil && strings.Trim(*tdata.Theme, " \t\r") != "" {
 			var unused map[string]string
 			if err := json.NewDecoder(strings.NewReader(*tdata.Theme)).Decode(&unused); err != nil {
-				return model.NewAppError("BulkImport", "app.import.validate_user_teams_import_data.invalid_team_theme.error", nil, err.Error(), http.StatusBadRequest)
+				return model.NewAppError("BulkImport", "app.import.validate_user_teams_import_data.invalid_team_theme.error", nil, "", http.StatusBadRequest).Wrap(err)
 			}
 		}
 	}