Add helpful error message when not returning a Dry::Monads::Result from step

[cllns]

When using dry-operation for the first time, I had a method with an if statement without an else, so it was returning nil when the condtion wasn't true. This messes up the step method because it always needs either a Dry::Monads::Result::Success or a Dry::Monads::Result::Failure object.

The error I was seeing was:

NoMethodError: undefined method `value_or' for nil

And (I think due to how we prepend the methods in this gem) there was no error pointing to dry-operation at all, the only stacktrace lines were within my app.

[waiting-for-dev]

Thanks @cllns, this is a very helpful addition. Just leaving a suggestion.

[timriley]

Looks good to me, once @waiting-for-dev's suggestion is done. 👍🏼 Thanks @cllns!

[cllns]

All set 🫡

[waiting-for-dev]

Thank you, @cllns 🙌