If an npm dependency can’t be found, and the package.json has a lock file, then Renovate will encounter lock file errors every time *any* dependency in that package.json has an update. Instead of raising PRs with an error, we instead now stop raising PRs and instead raise a config warning issue. Users can “dismiss” this by setting config option `updateLockFiles` to false.

Closes #1697

Instead of checking for config.repoIsOnboarded, just check for the onboarding PR and reverse the logic.

Closes #1339

This PR updates Renovate to detect config validation problems and (1) stop processing, and (2) either raise an Issue if already onboarded, or (2) update the onboarding PR to reflect the error if still onboarding.

Closes #1300