Content with errors stops site from building, blocking all other users

[btalb]

At the moment, if content is added which fails to build, the entire build process exits early.

This blocks updates for all other users as no new builds will successfully complete. Two things need to be addressed to resolve this:

1. Failures should be detected, and removed from the built content individually, so that the overall build can complete.
2. GitHub issues can then be automatically created in offending repos, or as a fallback an issue can be created here with the details for content without a repo attached (we can then manually ping the owner).