This CL makes proxy-related error messages more accurate and verbose.
(Exception: those used in deprecated functions in v8natives.js.)  Some of
the old error messages were simply wrong.

On the side, fix ShouldThrow semantics of JSProxy::SetPrototype and
JSProxy::DefineOwnProperty.

R=cbruni@chromium.org, jkummerow@chromium.org
BUG=v8:1543
LOG=n

Review URL: https://codereview.chromium.org/1527583002

Cr-Commit-Position: refs/heads/master@{#32836}