Try a less jarring exit strategy

Have client not provide a developer looking trace on abort.

confluent_client/confluent/client.py

@@ -263,6 +263,8 @@ class Command(object):
             else:
                 p = input('Command is about to affect {0} nodes, continue (y/n)? '.format(nsize))
             if p.lower() != 'y':
+                sys.stderr.write('Aborting at user request\n')
+                sys.exit(1)
                 raise Exception("Aborting at user request")