fix initial revision for policy posts & improve seed error reporting

[Oaphi]

This PR (followup for #1175) addresses the root cause of seeded posts not having an initial revision: owner user is required for the create_initial_revision validation callback to run successfully. I opted to assign the seeded posts to the system (-1) user as it seems the most fitting (let me know if y'all want me to guard against not finding the system user - I've cut the corner here as not having the system user at this point means something's very wrong already).

The PR also includes some error reporting improvements:

• Validation status when creating posts should be checked before reporting success (some of the seeded posts such as "Privacy Policy" fail the min title length validation, should be addressed by Skip min title validation for help/policy posts #1197);
• Seeds now collect the number of errored posts and report it after the run wraps up among other stats;