(PE-3659) Update Jetty shutdown processing for new TK release

This commit updates the project.clj dependency to a new Trapperkeeper
version which has support for calling service stops when an error occurs
during service startup.  The related Trapperkeeper changes were done in
the pull request associated with PE-3657.

This commit adds shutdown reason awareness into the the ring_app
repl.clj example, also made possible by changes from PE-3657.

This commit also updates the `shutdown` core function to support
shutdown being called when the Jetty server was not fully started --
essentially avoiding an exception that would have been thrown during
shutdown because of the missing Jetty server instance on the service
context.