mod_unified_push: Fix push error handling (fixes #2000) Use the error object that send_iq() passes as an argument to it's reject callback instead of attempting and failing to do the parsing in the callback itself.

This module lets you block connections to remote servers at the s2s
level.

``` {.lua}
modules_enabled = {
    -- other modules --
    "s2s_blacklist",

}
s2s_blacklist = {
    "proxy.eu.jabber.org",
}
```