Provision for application errors in ElementWriter::write_inner_content

[harmic]

write_inner_content is defined like this:

pub fn write_inner_content<F>(self, closure: F) -> Result<&'a mut Writer<W>>
where
    F: FnOnce(&mut Writer<W>) -> Result<()>,

This means that the closure can only return errors defined by quick_xml.

At present there doesn't seem to be any option within quick_xml::Error for wrapping an application generated error, so my options are limited if I want to bubble up some internal error.

One solution would be to provide another error variant, CustomError<dyn Box<std::error::Error>>, and ideally also provide a From implementation along with it so that ? can do its magic.

[Mingun]

I think, that we generally need to review our Error and DeError types because they looks random and inconsistent. It will be good to address this issue during that redesign.