Log an error when session fails to save.

Instead of a notice.

src/SaveSessionCommand.cc

@@ -69,8 +69,8 @@ void SaveSessionCommand::process()
       A2_LOG_NOTICE(fmt(_("Serialized session to '%s' successfully."),
                         filename.c_str()));
     } else {
-      A2_LOG_NOTICE(fmt(_("Failed to serialize session to '%s'."),
-                        filename.c_str()));
+      A2_LOG_ERROR(fmt(_("Failed to serialize session to '%s'."),
+                       filename.c_str()));
     }
   }
 }