Throw exception when variables do not exist in given dictionary.

[jeffijoe]

Currently I've got a sexy AssertVariableExists in the Base formatter that does one thing, and that is make the tests look good.

In other words, it's not being used, but it should. But I am thinking I shouldn't make each formatter be responsible for throwing, when I can do it in the MessageFormatter.FormatMessage before the actual formatter is being called.

Essentially, we want to know when we supplied bad data.

[jeffijoe]

Implemented in 07bde9a - v1 released.