Differential typeToString / smarter error truncation

[DanielRosenwasser]

Lots of the time, error truncation will kick in at the most inopportune time, and lose relevant properties that will be subsequently elaborated on. @RyanCavanaugh has been thinking about a smarter version of this where we can inform our string writers to be a little smarter and not throw away the relevant members of a union, or properties of an object.

But there are some areas that we need to kind of think through and be wary of.

1. don't repeat yourself: if you end up repeating a constituent of a union on the next line, then was it the best thing to focus on that part of the union on the prior line? You might avoid surprising a user ("where'd that type come from!?") but you're potentially not gaining anything. This leads us to
2. don't lose the right information: error messages don't always go down the most-appropriate path. If we try to over-focus on the types that the error decided to elaborate on, we shouldn't lose relevant information from other possible types.