http: clear content before reporting the failure

[Comment from commiter:] I have the feeling that the issue itself is
fixed for a while already as nowadays we have testcases involving a
webserver closing the connection on error (look for "closeOnError") and
no even remotely recent reports about it, but moving the content
clearance above the failure report is a valid change and shouldn't hurt.

Closes: #465572

methods/server.cc

@@ -770,8 +770,8 @@ int ServerMethod::Loop()
 	 // We need to flush the data, the header is like a 404 w/ error text
 	 case ERROR_WITH_CONTENT_PAGE:
 	 {
-	    Fail();
 	    Server->RunDataToDevNull();
+	    Fail();
 	    break;
 	 }