Catch error when unable to send DM (#909)

* fix: catch error when unable to send DM

* feat: empty success callback

* refactor(AutoPruneHelperRoutine): improve debug msg

* refactor(AutoPruneHelperRoutine): use logger formatter

Author: christolis

application/src/main/java/org/togetherjava/tjbot/features/help/AutoPruneHelperRoutine.java

@@ -163,7 +163,9 @@ private void pruneMemberFromRole(Member member, Role role, ForumChannel helpForu
         guild.removeRoleFromMember(member, role)
             .flatMap(any -> member.getUser().openPrivateChannel())
             .flatMap(channel -> channel.sendMessage(dmMessage))
-            .queue();
+            .queue(null, failure -> logger.debug(
+                    "Failed sending a DM to user ({}) while pruning them from a helper role.",
+                    member.getId()));
     }
 
     private void warnModsAbout(String message, Guild guild) {