Update error-handling in (at least) the config module

[chriskrycho]

Change all my instances of Result<T, String> to be Result<T, Vec<String>> instead. Collect errors to supply all validation errors to the user? And generalize that to all types? In the latter case, Vec<String> can become my internal error type—if not for the whole project, then at least for the config crate, where that's a super useful pattern for the validator(#12).