prepare_msg: return Err if no address is configured

Address is used to generate Message-Id.

Previously error toast was shown, but MsgId was returned from
chat::prepare_msg anyway. Now chat::prepare_msg returns an error.

error!() is removed, because FFI library shows errors via
.unwrap_or_log_default().

src/chat.rs

@@ -1015,7 +1015,7 @@ impl Chat {
                         );
                     }
         } else {
-            error!(context, "Cannot send message, not configured.",);
+            bail!("Cannot prepare message for sending, address is not configured.");
         }
         schedule_ephemeral_task(context).await;