Unsilence validation errors

[ddbeck]

Before this PR, we were silently ignoring all validation errors. 🤦

This fixes the issues in validate.ts and build.ts. It also fixes the individual validation errors.

One of these validation errors is a bit pedantic: some URLs get rejected for failing to match the uri string format. According to JSON Schema, a URI strictly follows RFC3986, which is ASCII only. Instead of rewriting those URIs, I chose to drop the uri format from the schema. Points in favor:

• It's not especially helpful to consumers, since we're making stronger guarantees about those URLs than mere URI-ishness.
• It's easier for authors to copy a link as it actually exists in the spec than to stop to %-encode those URLs.
• We get to drop a dependency.

I don't think this is a breaking change—it's still a string, we're not changing the meaning or scope of those strings, and no one has complained up to now that we're producing non-RFC3986 strings already—but I'd welcome a review of that aspect in particular.