Allow API to control messages returned to the user on submission

There are an increasing amount of usecases for the API being able to return custom messages upon
submission. Examples include being able to implement https://github.com/exercism/cli/issues/802 without cutting a new release, or being able to provide custom next steps depending on the type of exercise (mentored vs independent, core vs side, etc)

I'd like to return a message from the API, which then gets outputted at https://github.com/exercism/cli/blob/a86f82974270d2da2090a02437560897a216abc3/cmd/submit.go#L275

I think we can then also remove https://github.com/exercism/cli/blob/a86f82974270d2da2090a02437560897a216abc3/cmd/submit.go#L278-L281

Thoughts?