Replace Result type with cats.Validated about sbt-graphql

[felixbr]

I looked through the code to see if this issue is still needed.

So far it looks like Result is type Result[T] = Either[Failure, T] and Failure is just a wrapper around an error message.

Imo that's not perfect, but also not too bad atm. Replacing Either with ValidatedNel only makes sense for independent steps, but a lot of things are sequential in nature, so Either is fine as a default. If there are things where accumulating errors is possible, using ValidatedNel and converting the end result to Either is easy enough now (see Preprocessors.apply).

So there seem to be two questions left:

1. Is the type-alias worth it or would it be clearer to just use Either and ValidatedNel directly.
2. Would it make sense to have error-ADTs describing the different failure-cases as types (which would maybe also require (1) )

My personal thought is that an error-ADT would mostly be useful for testing, but the end-user of the plugin just wants decent error-messages, so the current solution could be sufficient. If we only transport error-messages we may as well keep the type-alias as it is now.

Just my 2 cents 🙂