Handle Invalid URIs in LinksChecker

`URI.parse` will fall over if the provided URI is invalid. This adds error handling to `LinksChecker` so badly formed URIs do not cause the checker to fall over (see [errbit error here](https://errbit.production.alphagov.co.uk/apps/53020d6c0da11585f10000e7/problems/53b67f4e0da115f5090021f4)).

Part of https://www.agileplannerapp.com/boards/173808/cards/3358