hackage-repo-tool bootstrap fails in a mysterious way if there are no tarballs

Steps to reproduce:

1. Run hackage-repo-tool bootstrap on a directory that doesn't exist. Get:
```
ezyang@sabre:~/Dev/labs/localrepo$ /srv/code/hackage-security/dist-newstyle/build/x86_64-linux/ghc-7.10.3/hackage-repo-tool-0.1.1/c/hackage-repo-tool/build/hackage-repo-tool/hackage-repo-tool bootstrap --keys keys --repo repo
/srv/code/labs/localrepo/repo: getDirectoryContents: does not exist (No such file or directory)
```

2. Create the directory and try again.
```
ezyang@sabre:~/Dev/labs/localrepo$ /srv/code/hackage-security/dist-newstyle/build/x86_64-linux/ghc-7.10.3/hackage-repo-tool-0.1.1/c/hackage-repo-tool/build/hackage-repo-tool/hackage-repo-tool bootstrap --keys keys --repo repo
/srv/code/labs/localrepo/repo/index: getDirectoryContents: does not exist (No such file or directory)
```

3. Create the directory and try again
```
ezyang@sabre:~/Dev/labs/localrepo$ /srv/code/hackage-security/dist-newstyle/build/x86_64-linux/ghc-7.10.3/hackage-repo-tool-0.1.1/c/hackage-repo-tool/build/hackage-repo-tool/hackage-repo-tool bootstrap --keys keys --repo repo
/srv/code/labs/localrepo/repo/01-index.tar: openBinaryFile: does not exist (No such file or directory)
```

That last error is incredibly oblique. You can solve it by adding a tarball to the `package` subdir. Totally non-obvious.

Reproduce the failure by running hackage-repo-tool bootstrap with an empty repository, creating the repo and repo/index directories as described. Trace the bootstrap path that handles the package directory and missing 01-index.tar. Done means an empty repository produces a clear, actionable result instead of the oblique missing-tarball error.

Written by the indexing model from the issue text.