-G/-A doesn't preserve gatherer errors

[brendankenny]

Internally they're real Errors, so they get serialized as {} :( This will obviously be an issue as errors won't show up in the -A stage. Presumably there will still be errors when audits try to use those artifacts, just not the ones you'd want to tell what's going on.

It would be nice to continue to use real Errors, so we should support saving and restoring errors in asset-saver if possible.

See skipped test TODO in runner-test.js which needs this in order to pass.