Fig config design 649

Design sketch for issue #649 
This and the follow tests for specific error conditions are great.  FWIW, a design sketch probably doesn't need this much detail though.  Demonstrating one error condition is probably okay to get the idea across.  In this case, it works out fine because I think you wrote exactly the tests that we're going to want.  If the design did need to change somehow then these tests would probably all need to get rewritten, though, so writing just enough to get the idea of how some part of the design will work and no more is probably a good idea.