reader: Dont return option for file

[rukai]

@Daniil-Golikov here is some extra feedback for #41 I figured it would be easiest to just demonstrate what I mean with a PR.

In general, we should never silently do nothing when we are expected to do something.
If we cannot carry out a task then we need to report to the user in some way that the task failed.
This is useful for both:

• the final end user - they need to know what went wrong so they can try again with better input.
• us as developers - if something goes wrong in our program it would be great if it told us what it was without a possibly lengthy debugging session.

In this case we have the partially implemented functionality around reading raw files.
Instead of returning a None, we should make some kind of error occur:

• Since we are in early stages of fft-compression project and there are no users yet our best option is a todo!() since that will direct developers directly to the line that needs further work via the stacktrace.
• If we did have users then we should be bubbling up an error or emitting an error! log instead. We already return a Result so we could just return a Result::Err instead of a Option::None.

Avoiding the option is also beneficial because we dont want the read_file API to be affected by the in progress nature of its current implementation. Since once its completed it wouldnt need the Option at all.

[Daniil-Golikov]

Thanks for the feedback on #41.

I agree about the importance of clear error handling.
Using todo!() at this stage makes sense.
Shifting from Option::None to Result::Err for read_file is a good suggestion.