Add another another error exercise

error_handling/errors3.rs

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+// This is a program that is trying to use a completed version of the
+// `total_cost` function from the previous exercise. It's not working though--
+// we can't call the `try!` macro in the `main()` function! Why not?
+// What should we do instead? Scroll for hints!
+
+use std::num::ParseIntError;
+
+fn main() {
+    let mut tokens = 100;
+    let pretend_user_input = "8";
+
+    let cost = try!(total_cost(pretend_user_input));
+
+    if cost > tokens {
+        println!("You can't afford that many!");
+    } else {
+        tokens -= cost;
+        println!("You now have {} tokens.", tokens);
+    }
+}
+
+pub fn total_cost(item_quantity: &str) -> Result<i32, ParseIntError> {
+    let processing_fee = 1;
+    let cost_per_item = 5;
+    let qty = try!(item_quantity.parse::<i32>());
+
+    Ok(qty * cost_per_item + processing_fee)
+}
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+// Since the `try!` macro returns an `Err` early if the thing it's trying to
+// do fails, you can only use the `try!` macro in functions that have a
+// `Result` as their return type.
+
+// The error that you get if you run this code is:
+
+// ```
+// error: mismatched types:
+// expected `()`,
+// found `std::result::Result<_, _>`
+// ```
+
+// which is saying that the expected return type of the `main` function is
+// the empty tuple, but we tried to return a `Result`-- and that's happening
+// in the implementation of `try!`. The `main` function never has a return type,
+// so we have to use another way of handling a `Result` within `main`.
+
+// Decide what we should do if `pretend_user_input` has a string value that does
+// not parse to an integer, and implement that instead of calling the `try!`
+// macro.