Defect messages: show the runtime values in expression?

[Udiknedormin]

When a contract is broken, only the original expression and its source are shown. It would be nice to also show the runtime values of the variables used in the expression, otherwise it's not really easy to debug e.g. loops.

Maybe some concept of "context" (e.g. loop counter) that should be passed alongside would be nice too. That way, contracts could show the circumstances of the failure more precisely.