I disagree to the selected answer. I would use the 503 code.
According to the RFC for HTTP/1.1:
503 Service Unavailable
The server is currently unable to handle the request due to a
temporary overloading or maintenance of the server. The implication
is that this is a temporary condition which will be alleviated after
some delay.
Yes, it should be an error, because the expected page will not appear.