[proxy] fixing for-in for proxies, fixing harmony/proxy.js tests, improving error messages and some drive-by fixes

BUG=v8:1543
LOG=n

patch from issue 1519473002 at patchset 1 (http://crrev.com/1519473002#ps1)

Review URL: https://codereview.chromium.org/1516843002

Cr-Commit-Position: refs/heads/master@{#32801}