plugin XEP-0045: don't fail on `item-not-found` with MAM: Some servers don't store permanently the whole history of MUC with MAM. As a result, an "item-not-found" stanza error can be received when message id used as reference doesn't exist anymore on the server. When this case happens, the history is retrieved in the same way as for a newly joined room (i.e. last 50 messages are requested).