provide a non monadic interface too

[craff]

Http_cookie should provide a non monadic interface too, raising an exception Error.
This is important as algebraic OCaml's 5.0 effect can replace monads.

Here is the design I propose:

• As Error are rare, the non monadic could be the default and the monadic function should be wrappers on top of them ?
• The functions could have the same names in 2 different modules ?

I could submit a PR, if we agree on the design first.