Allow uncaught exception messaging in Object.observe callbacks.

This also naturally handles pausing on uncaught exceptions in Object.observe callbacks.

R=adamk@chromium.org, yangguo@chromium.org, yurys@chromium.org
BUG=chromium:335660
LOG=Y

Review URL: https://codereview.chromium.org/692313003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25126 ce2b1a6d-e550-0410-aec6-3dcde31c8c00