Allow exporting of stored objects using delayed assignment

Might be useful when dealing with huge objects
Better error messages

Target name is always included in error message.  Will make things
way easier to track down.  Using withCallingHandlers rather than
tryCatch so that options(error=recover) works the way I'd like, but
this should not be a big deal for normal use (I hope).
Re-enable support for extra dependencies

In this case
```
targets:
  foo:
    rule: whatever()
    depends: [a, b, c]
```

We'll depend on a, b, and c, but pass none of them through to the
rule `whatever`.