Thank-you generation: shout at obvious mistakes

If unconsumed tokens are detected, throw errors.

A new drush job "find-unconsumed-tokens" is introduced, and make-thank-you is
enhanced to check itself.

Deployment:
* drush rr
* drush make-thank-you

Change-Id: I7b6bb4842ae49d26bfe9cccbd1d6f5c7cff2e1d6