Code review comment for lp:~edwin-grubbs/launchpad/bug-297833-invite-private-team

Summary
-------

This branch makes the "Constraint not satisfied" error message
clearer when someone tries to add a private team as a member
of another team.

I also noticed that the $team/+add-my-teams would OOPS if you
tried to add a private team as a member. I filtered out private
teams from the list of checkboxes, so that the error could not
occur on this page.

Tests
-----

./bin/test -vv -t team-views.txt

Demo and Q/A
------------

* Open http://launchpad.dev/~guadamen/+addmember
  * Enter "myteam" and click "Add member".
  * The error message should be:
    "A private team cannot become a member of another team."

* Open https://launchpad.dev/people/+newteam
  * Add a new team with the visibility set to private.
  * Open https://launchpad.dev/~guadamen/+add-my-teams
  * The newly created team should not appear in this list.