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They are scanning for vulnerabilities in found in common HTTP servers. You can change the port if you like, it might cut down some traffic. Although unless your server is extremely bandwidth constrained, I wouldn't worry about it. This kind of scanning does not typically generate a lot of traffic, because it is symmetric, in that the attacker has to use as much bandwidth as the responder, so attackers dont have a lot of incentive to generate a large number of requests.
And yes, an HTTP code other than 200 means the server replied with some kind of error message, you can see what they mean by looking up the list of HTTP codes on wikipedia.
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