node.rs: Implement fmt::Display and error::Error for node::Error
node.rs: Implement fmt::Display and error::Error for node::Error

We have an AttributeError struct that we use to hold an attribute's
name, and the actual error which that attribute caused.

node::Error in turn is an enum for parse errors, invalid values, etc.,
that each hold an AttributeError to describe the attribute in question.

GNOME/librsvg,GNOME/librsvg,GNOME/librsvg,GNOME/librsvg