Require a main function in all exercises

exercises/13_error_handling/errors2.rs

@@ -29,6 +29,10 @@ pub fn total_cost(item_quantity: &str) -> Result<i32, ParseIntError> {
     Ok(qty * cost_per_item + processing_fee)
 }
 
+fn main() {
+    // You can optionally experiment here.
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;