top-level: Fix splicing in the presence of evaluation errors

 - No more `or {}`, this was misleading at best since those values
   wouldn't be used unless the attr they are defined from was present
   anyways.

 - `tryEval` for get outputs. This ensures that if some derivations fail, they
   won't take out the others. This benefited from the `or {}` before, but that
   was never good enough. `tryEval` is strictly better.