Fix guide example code

[tolvanea]

(Beginner here, apologies if this is not valid issue.)

This guide example seems not to compile. There's this line:
use failure::err_msg;
but the called macro is:
return Err(format_err!("Input did not begin with $"));

So should the use statement be:
use failure::format_err; ?

Also there's other example, where is used

#[macro_use] extern crate failure_derive;

Could that be replaced with simpler use failure::Fail;? Then there would be no need to explicitly add failure_derive in Cargo.toml.

(As a personal opinion, the guide code could be updated to other new changes, like extern crate -> use.)