Simplify error handling pages (#851)

This makes the `use` statements more consistent and shortens some
variable names.

src/error-handling/deriving-error-enums.md

@@ -17,7 +17,7 @@ enum ReadUsernameError {
 }
 
 fn read_username(path: &str) -> Result<String, ReadUsernameError> {
-    let mut username = String::with_capacity(100);
+    let mut username = String::new();
     fs::File::open(path)?.read_to_string(&mut username)?;
     if username.is_empty() {
         return Err(ReadUsernameError::EmptyUsername(String::from(path)));

src/error-handling/dynamic-errors.md

@@ -4,7 +4,7 @@ Sometimes we want to allow any type of error to be returned without writing our
 all the different possibilities. `std::error::Error` makes this easy.
 
 ```rust,editable,compile_fail
-use std::fs::{self, File};
+use std::fs;
 use std::io::Read;
 use thiserror::Error;
 use std::error::Error;
@@ -14,8 +14,8 @@ use std::error::Error;
 struct EmptyUsernameError(String);
 
 fn read_username(path: &str) -> Result<String, Box<dyn Error>> {
-    let mut username = String::with_capacity(100);
-    File::open(path)?.read_to_string(&mut username)?;
+    let mut username = String::new();
+    fs::File::open(path)?.read_to_string(&mut username)?;
     if username.is_empty() {
         return Err(EmptyUsernameError(String::from(path)).into());
     }

src/error-handling/result.md

@@ -4,11 +4,11 @@ We have already seen the `Result` enum. This is used pervasively when errors are
 expected as part of normal operation:
 
 ```rust,editable
-use std::fs::File;
+use std::fs;
 use std::io::Read;
 
 fn main() {
-    let file = File::open("diary.txt");
+    let file = fs::File::open("diary.txt");
     match file {
         Ok(mut file) => {
             let mut contents = String::new();

src/error-handling/try-operator.md

@@ -19,8 +19,8 @@ some_expression?
 We can use this to simplify our error handing code:
 
 ```rust,editable
-use std::fs;
-use std::io::{self, Read};
+use std::{fs, io};
+use std::io::Read;
 
 fn read_username(path: &str) -> Result<String, io::Error> {
     let username_file_result = fs::File::open(path);