Better error handling (especially for validation)

[bernhold]

Right now, all we can do w.r.t. validating content, etc. is to emit error messages as part of the generated content. That has some value, but it also has its limits.

It would be nice to be able to emit warnings or errors into the logging stream of jekyll itself.

It would be nice to be able to raise error that would provide a useful error message and halt processing.

[bernhold]

We should also pull all validation up to the index pages rather than the individual content pages.

And we should reorganize the index pages to distinguish between those content pages that are ready to use and those that are not.

And we need to make all of the content pages more robust to missing data. They should do something sensible.

[bernhold]

More fundamentally, we should avoid jekyll failing to build due to missing data.