Don't explode on missing tags and optionally only index content with a given role

We have a fair bit of orphan content that doesn't have tags (but equally, is not published). We don't want Panopticon to explode on these, so have added another rescue block to prevent this.

Also, adding `ROLE=role` to the rake task will only index content with a given role, i.e:

    ROLE=dapaas rake panopticon:register

Handy as we're only dealing with Dapaas content for now.