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.