Use Result<(), ??> over ReturnCode

[daboross]

Making an issue here as a reminder.

It seems generally be more "rusty" to do this. The one disadvantage is having a two-byte return type size rather than one-byte, but that shouldn't be too large of a problem at all. We could even leave ReturnCode as a type alias for Result<(), ??>.

One question I'm not sure about is what to call the error portion. I'm thinking ErrorCode would work well, but just Error or GameError might also be good. I'm partially against Screeps* names because of duplication with screeps:: crate name.

[daboross]

Seems there are some coherence issues implementing TryFrom<Value> for Result<(), ErrorCode>. Not a show stopper, but means we'll need to do a bit more to actually implement this.

In the mean time I'm going to add a utility method to go from ReturnCode to a Result<(), ReturnCode> after it's been gotten.

[ASalvail]

With return code built as enums, this becomes obsolete. Instead of having a two element enum (Ok or Err) we have a ~20 elements enum.

I'd move to close this. @daboross

[daboross]

@ASalvail IIRC this has always been the case, there was never a time that ReturnCode wasn't a 20-element enum

The main advantages of Result that I was thinking of here are more from it being Result than from it being an enum in general. Like:

• being able to use the ? operator to propogate
• having combinator functions, like first_action().and_then(|()| second_action())
• integrating with other error types - you could have impl From<ErrorCode> for YourError and then use move_to()? in a function returning Result<_, YourError>, correctly propagating the error code
• other utility functions like Result::expect(message), or the implementation which allows Iterator::collect::<Result<Vec<_>, ErrType>>
• making it even clearer that all except Ok is an error. I'm mostly thinking of the fact that returning a Result is used almost universally among rust crates, and the pattern of combining the error variants with the success variants in one enum is probably unique to this one?