dub run <package> should search the registry on failure

Imho the following error message is a bit tricky to newcomers:

```
> dub run dlang-tour
Failed to find a package named 'dlang-tour'.
```

There are two easy solutions to this problem:

1) print a hint like 'Have you downloaded the package? Try `dub fetch <pkg>`
2) Automatically try to run `dub fetch` if `dub run` fails to find a package locally (preferred)

What do you prefer?
