Reland "Create errors (especially cancellation errors) internally to WebCore"

This reverts commit 98ebb3470a6348a0a8583b1d8a4b2a849d3b3177.

BUG=none
TEST=none

Review URL: https://chromiumcodereview.appspot.com/14409010

git-svn-id: svn://svn.chromium.org/blink/trunk@154432 bbb929c8-8fbe-4397-9dbb-9b2b20218538