Make 'svn checkout' report externals failures via its exit code,
like the export, list, switch, and update subcommands already do.

* subversion/svn/checkout-cmd.c
  (svn_cl__checkout): Make use of svn_cl__check_externals_failed_notify_wrapper.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1628398 13f79535-47bb-0310-9956-ffa450edef68