We must throw a SyntaxError only when failing to convert a string. In
the other cases we must throw a TypeError.

R=jkummerow@chromium.org

Bug: v8:6791
Change-Id: I802d8b6830b341f87e46e7de198af74ba95b8658
Reviewed-on: https://chromium-review.googlesource.com/752803Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49131}