Revamped external function result trees

* the root node of the result tree of each function is a domain element
** i.e. the root node of "read-xml-file" is "file", the root node of "read-directory" is "directory"
* the root node contains the result state of the function call encoded in a "result" attribute
** possible values are "success" and "error"
** the root node may contain additional attributes such as the target path of a called transformation
* the actual function result is contained within the child nodes of the function root node
** i.e. the XML file tree returned by "read-xml-file" is a child of the function root node
** if specific errors occured they are also returned as child nodes of the function root node
*** this is currently only the case for "transform" where transformation errors are returned as "error" value node childs of the function root node
* updated test cases accordingly

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