Log just one [info] message on successful XEP-0280 negotiation

Let `mod_carboncopy` log just [one][1] instead of three `info` messages when XEP-0280 (Message Carbons) support is enabled or disabled successfully.  On failure, log an additional `warning`.

[1]: https://github.com/processone/ejabberd/blob/5a29d56d94ba5d8d9504a0ac9c583a17d078fd96/src/mod_carboncopy.erl#L112