Fix constraints on get404 and getBy404
Fix constraints on get404 and getBy404

The constraints on `get404` and `getBy404` were overly powerful. They were
constrained by `PersistStore` and `PersistStoreUnique`, which is an alias for
`PersistStoreWrite...`. These only need `PersistStoreRead...` to accomplish
their job.

psibi/yesod,geraldus/yesod,s9gf4ult/yesod,yesodweb/yesod,jprider63/yesod,s9gf4ult/yesod,yesodweb/yesod,yesodweb/yesod,geraldus/yesod,geraldus/yesod,jprider63/yesod,psibi/yesod,jprider63/yesod,s9gf4ult/yesod,psibi/yesod