handle missing chain indicators in go model better

[csbrasnett]

as per this discussion, if you have a pdb which doesn't have chain IDs written into it (usually as a result of trjconv?), then the contact map server writes all chains as "Z", which then raises an issue when writing the go model.

This can usually be solved by going and writing the chains in yourself, regenerating the contact map, etc. But we should probably handle the issue better to indicate this. I'll add it to my 'sorting out the go model list'