Remove NoUserFound error constructor
Fixes #346 (closed).
While I was in the process of fixing the original issue (i.e. the presence of undefined
in the mapping between a backend internal error and a frontend error) I have realised that we were not even using that NoUserFound
constructor, so I have removed it: generally speaking it's better to be less specific, especially considering we are then forced to "bubble up" this error to the frontend, but we don't want to leak the fact a user exists or not in the platform.