Zavod validate step

[jbothma]

Adds validator interface used in zavod run and zavod validate.

zavod run skips validation for collections.

Both commands exit with non-zero status if an aborting validator triggers abort.

[jbothma]

this feels a tad icky, but rather than move the Statistics object somewhere more general, I think once we want more fancy assertions, we'll build a dedicated aggregator and leave the exporter alone.

[pudo]

no I like this a lot. Let's split it up early. The stats is part of our exposed interface so separating them makes a ton of sense to get ease of change for the validator.

[jbothma]

Do you mean I should implement statistics for the validator now, or use it as implemented here until we want a more powerful validator?

[pudo]

I'm very excited about how this is implemented, just wonder if we should have more clearly defined notions of how we'd abort a botched validation. Throwing a RuntimeException works, but it's not the most elegant thing we could do :)

[pudo]

Looks good to me, unless you want to do the Statistics fork now?

One thought: do we want to validate collections, too? Or just sources? Just in terms if how actionable the warnings are - it's all in the sources, no?

[jbothma]

oh yes definitely makes sense not to run this on collections, so we can keep publishing even when one source is misbehaving. I fix.