Validation of input

[dbrattli]

Add semantic types for validating input. https://fsharpforfunandprofit.com/posts/designing-with-types-more-semantic-types/

What to do if validation fails. We cannot throw an exception since F# programs will not expect exceptions. Thus we would need to build an error context to send down the pipeline that would give the same error as if we had sent the request to CDF. Thus we could just as well send it to CDF since it's referential transparent and handling the validation here would just be an optimisation, or?