IsExecutionTerminating returns true if a termination exception is
currently being propagated.  C++ code should not reenter V8 when
IsExecutionTerminating returns true.

Review URL: http://codereview.chromium.org/668052

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4016 ce2b1a6d-e550-0410-aec6-3dcde31c8c00