force a reason when calling set_msg_failed() (#3410)

* force a reason when calling `set_msg_failed()`

the string is displayed to the user,
so even _some_ context as "NDN without further details"
is better than an empty string.

* make clippy happy

* add CHANGELOG entry

src/dc_receive_imf.rs

@@ -2650,7 +2650,7 @@ mod tests {
             "shenauithz@testrun.org",
             "Mr.un2NYERi1RM.lbQ5F9q-QyJ@tiscali.it",
             include_bytes!("../test-data/message/tiscali_ndn.eml"),
-            None,
+            Some("Non-Delivery-Notification without further details."),
         )
         .await;
     }