`stack hoogle` fails in global project

```
~ $ stack hoogle Conduit
Run from outside a project, using implicit global project config
Using resolver: nightly-2016-08-06 from implicit global project's config file: /home/simon/.stack/global-project/stack.yaml
No Hoogle database yet. Automatically building haddocks and hoogle database (use --no-setup to disable) ...
Run from outside a project, using implicit global project config
Using resolver: nightly-2016-08-06 from implicit global project's config file: /home/simon/.stack/global-project/stack.yaml
Error parsing targets: The specified targets matched no packages.
Perhaps you need to run 'stack init'?
```

It looks like `stack hoogle` tries to build the haddocks but has no build target. Given that one can e.g. open haddocks (with `stack haddock --open`) from the global project, maybe `stack hoogle` could detect when it's in the global project and skip the haddock generation step?

If having a local project is a strict requirement, `stack hoogle` should at least fail more gracefully in the global project.

(Ping @chrisdone)