Custom Error as JSON from inside Handler

servant_custom_error.hs

{-# LANGUAGE DataKinds         #-}
{-# LANGUAGE DeriveGeneric     #-}
{-# LANGUAGE LambdaCase        #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE TypeFamilies      #-}
{-# LANGUAGE TypeOperators     #-}

module Api.Post
    (
    PostApi
    , postHandlers
    ) where

import           Control.Monad.Except
import           Control.Monad.IO.Class     (liftIO)
import           Data.Aeson
import           Data.ByteString            (ByteString)
import           Data.Maybe
import           Data.Pool                  (Pool, withResource)
import           Data.Text                  (Text)
import qualified Data.Text                  as T
import           Database.PostgreSQL.Simple
import           GHC.Generics
import           Network.HTTP.Types         (hContentType)
import           Servant

import           Models.Post


type PostApi = "post" :> Capture "id" Int  :> Get  '[JSON] BlogPost

postHandlers :: Pool Connection -> Server PostApi
postHandlers conn = blogPostDetailH
  where blogPostDetailH postId = withResource conn $ flip getPost postId

data ErrorTest = ErrorTest {status  :: Text
                          , message :: Text} deriving (Generic)
instance ToJSON ErrorTest

missingPostBody = encode ErrorTest { status = "missing", message = "Not found"}
jsonHeader = (hContentType, "application/json")

getPost :: Connection -> Int -> Handler BlogPost
getPost conn postId = do
  let q = "select * from post where id = ? and pubdate is not null"
  result <- liftIO $ query conn q (Only postId)
  case result of
    (x:_) -> return x
    []    -> throwError err404 { errBody = missingPostBody, errHeaders = [jsonHeader]}