Clarify reasons for Cache.match()/matchAll() failures

Tweak the tests to clarify why Cache.match failures are occurring:
* Re-putting request/response will fail, so clone() (issue 426146)
* Setting up the test data may fail due to put() bugs (call this out)

Doesn't change any FAILs to PASSes, but makes the causes more obvious.

BUG=374802
R=asanka@chromium.org

Review URL: https://codereview.chromium.org/678703003

git-svn-id: svn://svn.chromium.org/blink/trunk@184417 bbb929c8-8fbe-4397-9dbb-9b2b20218538