Add fail-on-error option to p:directory-list

[xatapult]

It would be helpful for some use-case to have a (boolean) fail-on-error option on p:directory-list. For instance when you want to know the contents of a directory but don't care whether it exists.

If fail-on-error is true, it would mean that when an (directory access) error occurs, it should just return the required root element, but without any child elements (maybe an error="true" attribute on the root element would be helpful as well here).