Fixes ENG-10886.
When user with given userID doesn't exist in Identity Service, we should check if the username is reserved, before setting the value to undefined. For reserved usernames, set thick thread support to false

1. On mobile, opened Friend list screen, searched for a user from the reserved usernames list (we have some of them on staging)
2. Tapped "Add" button
   • Before this diff: "Unknown error" alert caused by [Error: Cannot fetch user identity]
   • After this diff: User is added to friend list as non-supporting thick threads (given it exists on my local keyserver, otherwise invalid_user is thrown)

Also, console.logged values of isReserved, doesNotExist and usersSupportingThickThreads.get(userID) to confirm they're correct.