Show better CORS error message when status code is not set

When no response is received for a CORS request, Blink shows a misleading
message saying "No Access-Control-Allow-Origin header is present".
This change makes Blink say that "no valid response is received".

BUG=327834

Review URL: https://codereview.chromium.org/110273006

git-svn-id: svn://svn.chromium.org/blink/trunk@164744 bbb929c8-8fbe-4397-9dbb-9b2b20218538