Errors: New type called `Error` for all sorts of failures -- used to unify error handling and exit properly with source-location.

See the new type `Error` and function `Language.Haskell.Liquid.CmdLine.exitWithResult`.

You can `throw` an `Error` and that is what should be used so that the system always has some
place in the source for the user to fix, instead of the older ad-hoc error-star.