Postal marks messages as checked for spam even if it errored

As per #402 it seems messages are reported as checked and then publicly reported as "not spam" even if the spam check failed.

I would suggest a check needs adding here so that messages are marked as not inspected if a timeout or error has occurred
https://github.com/atech/postal/blob/6f29bc8d3f6f197d142724a7baee7376ff5fe8aa/lib/postal/message_db/message.rb#L499

I may be able to test and pull request this in later.

I presume a similar issue affects the virus check also.