Better message when illegal moves result from a binding

[nikomatsakis]

When you have a by-move binding in a match, it causes a move of the discriminant. It'd be nice if any errors resulting from this move identified the binding that caused the move. This requires some modifications to moves (to identify the binding and track it), liveness (to report the error in terms of the binding) and possibly borrowck (which also can report errors related to illegal moves)

[catamorphism]

Seems not critical for 0.7. Nominating for milestone 5, production-ready.

[graydon]

Accepted for production ready