rss_error() is not returning a proper HTTP response code

While testing MDL-33514 I discovered that rss_error is returning a 200 response code when generating errors. This isn't really correct and it'll mean things like google reader will continually request broken urls. We need to change that.

wget -O /dev/null -S http://dan.moodle.local/integration/rss/file.php/a_made_up_url
-2012-06-06 23:49:41- http://dan.moodle.local/integration/rss/file.php/a_made_up_url
Resolving dan.moodle.local... 127.0.0.1, 127.0.0.1
Connecting to dan.moodle.local|127.0.0.1|:80... connected.
HTTP request sent, awaiting response...
HTTP/1.1 200 OK
Date: Wed, 06 Jun 2012 15:49:46 GMT
Server: Apache/2.2.21 (Unix) DAV/2 PHP/5.3.10
X-Powered-By: PHP/5.3.10
Content-Disposition: inline; filename="rss.xml"
Cache-Control: private, must-revalidate, pre-check=0, post-check=0, max-age=0
Expires: Thu, 01 Jan 1970 00:00:00 GMT
Pragma: no-cache
Last-Modified: Wed, 06 Jun 2012 15:49:46 GMT
Accept-Ranges: none
Content-Length: 864
Keep-Alive: timeout=3, max=100
Connection: Keep-Alive
Content-Type: application/xml
Length: 864 [application/xml]
Saving to: `/dev/null'