(PE-3140) Wrap webserver start in shutdown-on-error

This commit wraps the `start-webserver` call during the `start` phase of
the webserver-jetty9 service in a `shutdown-on-error` call.  This allows
the Jetty server instance to be persisted to the service context and,
therefore, have a `stop` call for the service stop the Jetty server
instance.  Stopping of the Jetty server instance is necessary in order
for the JVM process in which the server was started to gracefully exit.
(PE-3140) Catch `start-webserver` errors in `start`, calling `shutdown`

This commit changes the implementation of error handling for the call to
`start-webserver` during the start phase of the service to call
`shutdown` explicitly.  The implementation in the previous commit relied
upon the Trapperkeeper framework to invoke `shutdown` implicitly when
the service was being shut down -- signalled through the exception being
caught in a `shutdown-on-error` block.
(PE-3140) Removed unnecessary :ShutdownService shutdown-on-error reference

This commit removes an unnecessary reference to the shutdown-on-error
function in the :ShutdownService from the jetty9-service defservice.
Merge pull request #27 from camlow325/maint/master/PE-3140-handle-jetty-startup-errors

(PE-3140) Wrap webserver start in shutdown-on-error