Various API-related simplifications.

   * Simplified default fatal error handler.

   * Simplified Context::Exit and friends.

   * Inline API_ENTRY_CHECK.

R=dcarney@chromium.org

Review URL: https://codereview.chromium.org/137963002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18577 ce2b1a6d-e550-0410-aec6-3dcde31c8c00