Handle errors more sanely

[sdboyer]

trafficstars

The simplest thing we can do to make errors sane is:

• Have one/a couple specific error interfaces that differentiate between errors as true satisfiability failures vs. mechanical errors (i.e. those coming from the SourceManager)
• In service of the above, have a mechanism for wrapping up the errors from a SourceManager as their own type...ish?

Also, a third thing. I think. I forgot

[fabulous-gopher]

This issue was moved to golang/dep#450