mod_rest: Improve error handling during format mapping Prevents e.g. a nil, error return going directly into e.g. json encoding, resulting in "null" being returned. Further handling improvements down the line is needed.

This module blocks configured queries that change server state.

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

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