Improve serialization failure reporting

[neoeinstein]

Prefer JsonResult<'a> as the return type for deserialization.

• Create a type, JsonError, consisting of all potential JSON deserialization failures
  • Include a recursive Tag case to allow structured reporting
  • Include a OtherDeserializationError to allow consumers to extend with their own failures
• Modify JsonResult<'a> to have a JsonError list in the Error case
• Define functor and applicative functions for JsonResult<'a>
• Add a formatError : JsonError -> string function
• Add a JsonResult.toOption function
• Add a JsonResult.toChoice function

[kolektiv]

Perhaps also a JsonResult.toOption as well, for when you really don't care why it failed, just that it did.