Add suggestion to use "no-cors" with Fetch fails CORS check.

LayoutTests/http/tests/fetch/chromium/error-messages-expected.txt

-CONSOLE ERROR: Fetch API cannot load http://localhost:8000/fetch/resources/doctype.html. No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'http://127.0.0.1:8000' is therefore not allowed access.
+CONSOLE ERROR: Fetch API cannot load http://localhost:8000/fetch/resources/doctype.html. No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'http://127.0.0.1:8000' is therefore not allowed access. Alternatively, if you want an opaque response, switch to The Fetch API in 'no-cors' mode.
 CONSOLE ERROR: Fetch API cannot load http://localhost:8000/fetch/resources/redirect-loop.php?Count=100&ACAOrigin=*. Redirect failed.
 CONSOLE WARNING: Request.context is deprecated and will be removed in M46 (see: https://www.chromestatus.com/feature/5534702526005248).
 This is a testharness.js-based test.
 PASS fetch() with 200 should not output error messages 
 PASS fetch() with 404 should not output error messages 
 PASS net::ERR_TOO_MANY_REDIRECTS should not output error messages 
 PASS net::ERR_INVALID_CHUNKED_ENCODING should not output error messages 
 PASS A didFailAccessControlCheck() error should output an error message 
 PASS A didFailRedirectCheck() error should output an error message 
 PASS A deprecation warning should be output for Request.context 
 Harness: the test ran to completion.