fix(backend): return 404 if custom asset files are missing

assets.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"errors"
+	"fmt"
 	"html/template"
 	"net/http"
 	"os"
@@ -81,7 +82,7 @@ func serverFileOrEmpty(path string, contentType string, c *gin.Context) {
 		return
 	}
 	if _, err := os.Stat(path); os.IsNotExist(err) {
-		c.Data(200, contentType, nil)
+		c.Data(404, contentType, []byte(fmt.Sprintf("%s not found", path)))
 		return
 	}
 	c.File(path)

assets_test.go

@@ -33,15 +33,15 @@ func TestCustomizationAssets(t *testing.T) {
 		{
 			customJS: "foo/bar/custom.js",
 			path:     "/custom.js",
-			code:     200,
-			body:     "",
+			code:     404,
+			body:     "foo/bar/custom.js not found",
 			mime:     "application/javascript",
 		},
 		{
 			customCSS: "foo/bar/custom.css",
 			path:      "/custom.css",
-			code:      200,
-			body:      "",
+			code:      404,
+			body:      "foo/bar/custom.css not found",
 			mime:      "text/css",
 		},
 		{