No instance for Servant.Foreign.Internal.NotFound

I am currently trying to generate JS code for my API and I have this:

```
angular.hs:9:8:
No instance for Servant.Foreign.Internal.NotFound
arising from a use of ‘writeJSForAPI’
In the expression:
writeJSForAPI
(Proxy :: Proxy MyAPI)
(angular defAngularOptions)
"./my_api.js"
In an equation for ‘main’:
main
= writeJSForAPI
(Proxy :: Proxy MyAPI)
(angular defAngularOptions)
"./my_api.js"
```

The API in question is huge and I'm not sure what exactly is causing this.
Perhaps we should change `NotFound` to something else to save the context.
E.g. it can be something like

```
class NotFound _what _where
```

So that I would at least receive

```
No instance for Servant.Foreign.Internal.NotFound JSON '[MyContentType]
```