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 blocks configured queries that change server state.

E.g. to make vCard storage read-only:

``` lua
readonly_stores = {
	vcard = { "vcard-temp", "vCard" };
}
```