Require a main function in all exercises

exercises/13_error_handling/errors1.rs

@@ -9,6 +9,10 @@
 // Execute `rustlings hint errors1` or use the `hint` watch subcommand for a
 // hint.
 
+fn main() {
+    // You can optionally experiment here.
+}
+
 pub fn generate_nametag_text(name: String) -> Option<String> {
     if name.is_empty() {
         // Empty names aren't allowed.

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::*;

exercises/13_error_handling/errors4.rs

@@ -19,6 +19,10 @@ impl PositiveNonzeroInteger {
     }
 }
 
+fn main() {
+    // You can optionally experiment here.
+}
+
 #[test]
 fn test_creation() {
     assert!(PositiveNonzeroInteger::new(10).is_ok());

exercises/13_error_handling/errors6.rs

@@ -54,6 +54,10 @@ impl PositiveNonzeroInteger {
     }
 }
 
+fn main() {
+    // You can optionally experiment here.
+}
+
 #[cfg(test)]
 mod test {
     use super::*;