vg autoindex fails with the wrong error on compressed GFA

If you `vg autoindex -w sr-giraffe -g graph.gfa.gz`, it can fail with:
```
[IndexRegistry] Checking for haplotype lines in GFA.
error[IndexRegistry] W-line does not have tab following line type
```

It should instead report that vg does not currently support importing compressed GFA.

We should also consider how to actually support importing compressed GFA.