Sign up errors displayed as an array of strings

If you try to sign up a new user account, and the username already exists, the error message is this:

["has already been taken"]

In spanish:

["ya está en uso","ya está en uso","ya está en uso"]

I think this message is coming from the android app, and not the webapp.

(from redmine: created on 2016-10-24)

blocked by !28 (merged)