HTML and plain text for new post mail notifications

src/routes/admin/newsletters.rs

@@ -112,7 +112,7 @@ pub async fn publish_newsletter(
 
 fn validate_form(form: &BodyData) -> Result<(), &'static str> {
     if form.title.is_empty() {
-        return Err("The title was empty");
+        return Err("The title was empty.");
     }
     if form.html.is_empty() || form.text.is_empty() {
         return Err("The content was empty.");