Consistently use err for the error value (#599)

src/error-handling/try-operator.md

@@ -24,17 +24,15 @@ use std::io::{self, Read};
 
 fn read_username(path: &str) -> Result<String, io::Error> {
     let username_file_result = fs::File::open(path);
-
     let mut username_file = match username_file_result {
         Ok(file) => file,
-        Err(e) => return Err(e),
+        Err(err) => return Err(err),
     };
 
     let mut username = String::new();
-
     match username_file.read_to_string(&mut username) {
         Ok(_) => Ok(username),
-        Err(e) => Err(e),
+        Err(err) => Err(err),
     }
 }