Change the type of the CantResolveAlts constructor.

I changed the type of the `CantResolveAlts` constructor from `[String] -> Err` to `[Name] -> Err`. This way, we can pretty-print the names when we can't resolve alternatives. Anyway, most uses of `CantResolveAlts` involved projecting a `Name` down to a `String` using `show`.

This was done to address https://github.com/idris-lang/Idris-dev/issues/1267#issuecomment-44655256.

There is one place, however, (shown below) where we may need to watch out...