Don't route PEP error messages to clients

Drop PEP error messages, as [mandated][1] by XEP-0163.  ejabberd [did this][2] in the past, but the code [got][3] [lost][4].

[1]: http://xmpp.org/extensions/xep-0163.html#notify-addressing
[2]: https://github.com/processone/ejabberd/commit/839b406903e49170fd266e8143249c926f552612
[3]: https://github.com/processone/ejabberd/commit/f0a848ef45b8be023e757483a7d04a5b8b018f12
[4]: https://github.com/processone/ejabberd/commit/fd828c3e9bd57c1ac0e0e4b118b266833519ec18