Fix wrong variable name.
Represent fetchers as maps with a :fetch-fn.
Store errors in the agent state.

That way we can inspect them with the `/status` resource.
Support serving data resources pretty printed.

Now we can inspect the agent states neatly as they can be pretty printed
and contain the agent state + the last error.
Remove error and add fetch date on success.
Try to make refetching possible.

Theoretically this could work, as the next fetch is scheduled before the
current fetch is executed, but i'd have to test this.