core: clear group content on deletion, break transaction to prevent error on Android, more logs (#892)

* core: log group deletion

* clear group content, break transaction, add logs

src/Simplex/Chat/Util.hs

@@ -11,3 +11,7 @@ safeDecodeUtf8 = decodeUtf8With onError
 
 uncurry3 :: (a -> b -> c -> d) -> ((a, b, c) -> d)
 uncurry3 f ~(a, b, c) = f a b c
+
+lastMaybe :: [a] -> Maybe a
+lastMaybe [] = Nothing
+lastMaybe xs = Just $ last xs