Display implementation should show the error's Display, not just the description

[joshtriplett]

Given the following invocation of error_chain!:

error_chain! {
    types {
        Error, ErrorKind, ChainErr, Result;
    }
    links {}
    foreign_links {
        git2::Error, Git2, "git error";
    }
    errors {
    }
}

Producing an error from a git2::Error and then printing that error just produces "git error", without actually printing the underlying git2::Error. The Display implementation for an error produced from a foreign link should forward to the Display implementation for the linked error.

[brson]

Thanks for moving this over to the new repo. I agree this would be better. Help wanted.