Make so vili::validator::validate_tree validates the whole tree

[Sygmei]

Right now, as soon as vili::validator::validate_tree encounters an exception, it stops crawling the tree and raises the exception.

This is acceptable for now but when having huge files, it would be super useful to be able to validate all the issues at once.

The way it could be implemented is by returning a vili::node that contains all validation failures, the validate_tree function would become nothrow.

This could be especially useful in IDEs integration (being able to validate a vili file by using the official schema).