Make add_host() really return an error code.

Make add_host() really return an error code.

Now that add_host() *may* return an error, it is kind of confusing
(and actually leads to errors) that it also returns the number of
hosts added to the value/result "hosts" array.  Since the caller
really checks add_host()'s return value now, a successful lookup that
returns one host added will be treated as an error :)