[import] Handle 40x response as an error.

Resource invokes its client in slightly different ways for
file:// and http:// URLs: It deson't call responseReceived
for non-existent local URLs. This difference caused crbug.com/336609.

This change adds a check against status code in responseReceived() so that
the loader doesn't create document for error response.
The client of RawResource needs to handle these low level details.

TEST=import-onerror.html,import-onload.html
BUG=336609
R=dglazkov@chromium.org, dominicc@chromium.org

Review URL: https://codereview.chromium.org/145493011

git-svn-id: svn://svn.chromium.org/blink/trunk@166098 bbb929c8-8fbe-4397-9dbb-9b2b20218538