Service Worker Cache: Simplify exception/rejection processing

Rather than explicitly catching exceptions and generating promise
rejections, let the bindings code do the work.

R=jkarlin
BUG=392621

Review URL: https://codereview.chromium.org/694083002

git-svn-id: svn://svn.chromium.org/blink/trunk@184886 bbb929c8-8fbe-4397-9dbb-9b2b20218538