Friendlier, first person error messages

[nikomatsakis]

One of Dada's precepts is that it should be fun to use and easy to learn. There's been research that shows that a more "first person" compiler is more accessible. This would mean giving errors like "I expected to see a X, but I see Y instead". I think this would be a good fit for Dada. In the playground we could have Tzara speaking the errors with a speech bubble :)

Anyway, this is low priority, but I wanted to record it.