set_username: replace some unwraps with recovery paths + logging

i don't actually know much about this path: an `expect` might make more
sense. i'm just trying to make this more debuggable.

src/main.rs

@@ -160,9 +160,15 @@ impl Handler {
 
     fn set_username(&mut self) {
         if let Some(user_list) = lightdm::UserList::get_instance() {
-            if let Some(name) = user_list.get_users().first().unwrap().get_name() {
+            match user_list.get_users().first() {
-                let name_str = Box::<str>::from(name);
+                Some(user) => match user.get_name() {
-                self.user = Some(String::from(name_str));
+                    Some(name) => {
+                        let name_str = Box::<str>::from(name);
+                        self.user = Some(String::from(name_str));
+                    },
+                    None => log::warn!("user.get_name() returned None"),
+                },
+                None => log::warn!("user_list.get_users() returned empty"),
             };
         }
     }