config.rs

extern crate serialize;
extern crate std;
extern crate collections;

use self::serialize::json;
use self::serialize::json::Json;
use self::std::io::File;
use self::collections::TreeMap;

pub fn load_config_map() -> Result<TreeMap<~str, Json>,~str> {
    let config: Json = match json::from_reader(&mut File::open(&Path::new("config.json"))) {
        Ok(v) => v,
        Err(e) => return Err(format!("Failed to decode config: {}", e))
    };
    match config {
        json::Object(obj) => Ok(*obj),
        _ => Err("Config isn't an object!".to_owned())
    }
}

Note, the ~str error type is kind of unfortunate because it's not machine-parsable, although you could present it to the user. A better approach is to define an enum with error codes and return that instead.