The user wants to load an xml document, i have to validate that the structure is

There's no out of the box to validate an XML. The xpathsnippet() function will fail and throw an error if the XML is not well-formed but that won't help with your requirement since that will mean the node will fail and you won't be able to "catch" and handle that exception.

Since this is an XML related requirement you should be able to find libraries online or even sample code to validate an XML in Java and then build your own custom function. Here's one example stackoverflow.com/.../whats-the-best-way-to-validate-an-xml-file-against-an-xsd-file