mod_bookmarks2: Ignore failure to delete nothing (thanks Ge0rG) Attempting to delete all bookmarks when the bookmarks node does not even exist logged a fairly harmless error that we now ignore.

This module blocks configured queries that change server state.

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

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