Preserve message when rethrowing exception

A new message was always generated if there is a the top-most verbose
TryCatch, even when rethrowing an exception from a TryCatch that is going
out of scope, and we already have a message.

BUG=v8:3583
LOG=Y
R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/587703002

Patch from Jens Widell <jl@opera.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24152 ce2b1a6d-e550-0410-aec6-3dcde31c8c00