Twitter is just a piece of nonsense. And morons sending abuse on it are, well, morons..........
We can all be mad at Langfield for his mistakes be sending him or anyone on our team death wishes is a disgrace.
To answer your bolded point, all keepers make mistakes, it just seems that Langfield keeps making them, again and again. He seems to get a lot of leeway from the coaching staff because he's a great shot stopper. Marvellous, except his kicking, coming from crosses, communication and general presence in the box are all sadly below par.
J Brown is, I think, a bit better than Clangers, but he's no Leighton/Snelders/Kjaer or even Burridge.