When ajax requests fail, the failure message should remain visible until the user dismisses it

Use any of open quickmod issues, e.g. http://tracker.phpbb.com/browse/PHPBB3-10802, to produce an ajax failure. The error message, if it is displayed, is automatically hidden after ~100 ms. Therefore the user effectively receives no error message when ajax requests fail.

If an error message is displayed, it must be kept visible until the user has read it and dismissed it.