fail - a new error handling story

Contains three parts (only a sketch right now):

• A Fail trait and Error type wrapper, which acts as a dynamically dispatched, open sum type
• match_err!, a macro to give match-like syntax for downcasting the Error type
• derive(Fail), a derive for making your type an error type

Example use case:

#[macro_use] extern crate fail;
#[macro_use] extern crate fail_derive; // haven't actually implemented this yet

#[derive(Debug, Fail)]
#[error_msg("something went wrong {}", self.message)]
struct CustomError {
    message: String,
}

fn main() {
    use std::io;

    let err = run_program();
    match_err!(err => {
        io::Error:   err    => { println!("IO error: {}", err) }
        CustomError: err    => { println!("internal error: {}", err) }
        final:       _      => { panic!("should not have another kind of error") }
    });
}