Closes#208.

Queryingapi/v2/users/{ID} returns a valid response for deleted users, as deleted users incoderd are merely tombstoned. To handle this, we perform an additional query by username. If the user has been deleted, the username will be available, or belong to a user with a different ID, in which case we can mark the user resource as deleted.

Also has theisNotFound check include the specific response for when a user does not exist:

sdkErr.StatusCode() == http.StatusBadRequest && strings.Contains(sdkErr.Message, "must be an existing uuid or username")