Fix returned HTTP response for some specific cases

[ayoubfaouzi]

• [x] Get/files/sha256?fields=x always includes comments_count, we should omit that field.
• [x] Get /files/sha256?fields=not_exist return internal error, we should return bad request or something like that.
• [ ] When there is no activity, the log shows some internal server error.
• [ ] Following a non existing user returns ok

[ayoubfaouzi]

#76

[ayoubfaouzi]

TODO:

• check filter allowed for user entity when using lookups, maybe redundant.