treexml::Error does not implement std::error::Error

[eatnumber1]

Per https://doc.rust-lang.org/book/first-edition/error-handling.html#the-error-trait, treexml::Error should implement std::error::Error, but it does not. This prevents converting a treexml::Error into a Box<std::error::Error>, which is useful as a generic return type from a method that can fail due to multiple reasons.