Return the status nodeeventlog worked out

Every error branch set exitcode and the script then ran off the end, so
a node that could not be read, or a log source that matched nothing,
still exited 0.

confluent_client/bin/nodeeventlog

@@ -230,3 +230,5 @@ if options.lines and not listmode:
                             print('{0}: {1}'.format(node, format_event(evt)))
                 else:
                     print('{0}: {1}'.format(node, format_event(evt)))
+
+sys.exit(exitcode)