Helper for displaying all causes in the entire error chain about failure

[lilianmoraru]

The error-chain is my favorite feature in error-chain.
I remember rustup failing to download a file from behind the company's proxy and instead of getting the usual misleading error: No such file or directory, I got a very clear chain of errors explaining that:

1. It failed to access the link
2. Because it failed to access the link, it failed to download the file
3. Because it failed to download the file, it does not exist

It made it extremely easy for me to fix the root cause.

Edit(the rustup output when I remove the SOCKS4 proxy):

error: could not download file from 'https://static.rust-lang.org/rustup/release-stable.toml' to '/tmp/rustup-update.T7Ry8gjuIytx/release-stable.toml'
info: caused by: error during download
info: caused by: [7] Couldn't connect to server (Failed to receive SOCKS4 connect request ack.)

[epage]

For reference, with #54 here is an approximation (meaning I haven't built it) of error-chain:

// Decide which command to run, and run it, and print any errors.
if let Err(err) = run(&args) {
    let mut stderr = io::stderr();
    let mut causes = err.causes();
    writeln!(stderr, "Error: {}", causes.next().expect("`causes` to at least contain `err`"))
        .expect("unable to write error to stderr");
    for cause in causes {
        writeln!(stderr, "Caused by: {}", cause)
            .expect("unable to write error to stderr");
    }
    // The following assumes an `Error`, use `if let Some(backtrace) ...` for a `Fail`
    writeln!(stderr, "{:?}", backtrace)
        .expect("unable to write error to stderr");
    process::exit(1);
}

[withoutboats]

I've just merged #54, which implements a causes iterator. I think something like this would be adequate for your use case for now:

let mut stderr = io::stderr();
for fail in err.causes() {
    let _ = writeln!(stderr, "{}", fail);
}