hint to the log messages on error

src/ulo-storage-applications/src/main/java/info/mathhub/uloapi/html/Errors.java

@@ -17,6 +17,8 @@ public class Errors {
     /**
      * Singleton instance of the Freemarker engine for use with errors.
      * Used in {@link Errors#createError}.
+     *
+     * This object is not safe for concurrent use by multiple threads.
      */
     private static final FreeMarkerEngine freemarker = new FreeMarkerEngine();
 

src/ulo-storage-applications/src/main/resources/spark/template/freemarker/error.flt

@@ -7,6 +7,11 @@
 <#macro page_main>
     <h2>Error ${http_status}: ${http_description}</h2>
 
+    <p>
+        If you are the administrator of this instance, consider looking at the log
+        output.
+    </p>
+
     <#if message?has_content>
         <code>
             ${exception_message}