Use elaborateFailure in getFieldOptional'

[LucianU]

What does this pull request do?

This changes getFieldOptional' to use elaborateFailure, just like its sister function getFieldOptional.

How should this be manually tested?

import Data.Argonaut.Decode (decodeJson)
import Data.Argonaut.Decode.Combinators (getFieldOptional')
import Data.Argonaut.Parser (jsonParser)
import Data.Maybe
import Data.Either
import Prelude

jsonParser "{\"age\": \"26\"}" >>= decodeJson >>= (\obj -> (obj `getFieldOptional'` "age") :: Either String (Maybe Int))

Without the change, the result is (Left "Value is not a Number").
With the change, the result is (Left "Failed to decode key 'age': Value is not a Number")

[thomashoneyman]

Thanks! This can be seen in release v5.1.3.