Panic on unreachable AWS SES code

This is clearer to those reading code on expectations.

src/mail.rs

@@ -704,7 +704,7 @@ async fn send_with_selected_transport(email: Message) -> EmptyResult {
         }
 
         #[cfg(not(ses))]
-        err!("Failed to send email", "Failed to send email using AWS SES: `ses` feature is not enabled");
+        unreachable!("Failed to send email using AWS SES: `ses` feature is not enabled");
     } else {
         match smtp_transport().send(email).await {
             Ok(_) => Ok(()),