Avoid mentioning source types of object literals in error messages

Whenever a type that originates from an object literal is encountered, we should avoid writing out the whole type. It's rarely useful, and can get painful in a lot of contexts (think React Native styles objects). ```diff - Type '{ goobledeyGock: string, tickTock: number, wakkaFlocka: { stuff: string } }' is not assignable to 'OneType | AnotherType'. + This object is not assignable to 'OneType | AnotherType'. ```