Improve error messages.

[eaburns]

Check errors should include the offending code. That's what gc does: http://play.golang.org/p/SJdh4XNzCB

The current error messages for bad assignments are particularly poor: "bad assignment." Not a helpful message.

[eaburns]

This isn't a valid issue yet. It's referring to stuff in the un-merged check branch. It will be fixed before that branch is merged.

[eaburns]

This isn't invalid any more, because I want to merge my check branch before I bother fixing this.