ドキュメントが古いせいか完全な一致ではありませんが、`Description` として表示されるメッセージの文言だと思います。
```
$ curl -v -H "Host: x.yimg.com"  http://s.yimg.com/
* About to connect() to s.yimg.com port 80 (#0)
*   Trying 203.84.208.51...
* Adding handle: conn: 0x7fd252804000
* Adding handle: send: 0
* Adding handle: recv: 0
* Curl_addHandleToPipeline: length: 1
* - Conn 0 (0x7fd252804000) send_pipe: 1, recv_pipe: 0
* Connected to s.yimg.com (203.84.208.51) port 80 (#0)
> GET / HTTP/1.1
> User-Agent: curl/7.30.0
> Accept: */*
> Host: x.yimg.com
> 
< HTTP/1.1 404 Not Found on Accelerator
< Date: Sat, 14 Dec 2013 13:36:28 GMT
< Connection: close
< Via: http/1.1 l6.ycs.krx.yahoo.com (ApacheTrafficServer/4.0.2)
* Server ATS/4.0.2 is not blacklisted
< Server: ATS/4.0.2
< Cache-Control: no-store
< Content-Type: text/html; charset=utf-8
< Content-Language: en
< Content-Length: 304
< 
<HTML>
<HEAD>
<TITLE>Not Found on Accelerator</TITLE>
</HEAD>

<BODY BGCOLOR="white" FGCOLOR="black">
<H1>Not Found on Accelerator</H1>
<HR>

<FONT FACE="Helvetica,Arial"><B>
Description: Your request on host "<EM>x.yimg.com<EM>" was not found.
Check the location and try again.
</B></FONT>
<HR>
</BODY>
* Closing connection 0
```