Improve error/success message for dcos-oauth dcos_add_user
Improve error/success message for dcos-oauth dcos_add_user

* A success message will be printed to stderr and exit code 0 when adding user is successful
* An error message will be printed to stderr and exit code 1 when connection to zk times out
* An error message will be printed to stderr and exit code 1 if unable to create the `/dcos/users/` base node
* An error message will be printed to stderr and exit code 1 when attempting to add a user that already exists

mesosphere-mergebot/dcos,surdy/dcos,darkonie/dcos,asridharan/dcos,mellenburg/dcos,asridharan/dcos,dcos/dcos,dcos/dcos,branden/dcos,dcos/dcos,amitaekbote/dcos,vishnu2kmohan/dcos,lingmann/dcos,mellenburg/dcos,vishnu2kmohan/dcos,BenWhitehead/dcos,mnaboka/dcos,mesosphere-mergebot/mergebot-test-dcos,mnaboka/dcos,BenWhitehead/dcos,asridharan/dcos,amitaekbote/dcos,jeid64/dcos,mellenburg/dcos,vishnu2kmohan/dcos,mesosphere-mergebot/dcos,dcos/dcos,branden/dcos,branden/dcos,mesosphere-mergebot/dcos,kensipe/dcos,GoelDeepak/dcos,mesosphere-mergebot/mergebot-test-dcos,mellenburg/dcos,branden/dcos,mesosphere-mergebot/mergebot-test-dcos,mesosphere-mergebot/dcos,GoelDeepak/dcos,kensipe/dcos,xinxian0458/dcos,surdy/dcos,jeid64/dcos,jeid64/dcos,amitaekbote/dcos,mnaboka/dcos,mnaboka/dcos,mesosphere-mergebot/mergebot-test-dcos,darkonie/dcos,darkonie/dcos,dcos/dcos,mnaboka/dcos,kensipe/dcos,lingmann/dcos,vishnu2kmohan/dcos,amitaekbote/dcos,xinxian0458/dcos,xinxian0458/dcos,jeid64/dcos,asridharan/dcos,lingmann/dcos,GoelDeepak/dcos,xinxian0458/dcos,darkonie/dcos,surdy/dcos,GoelDeepak/dcos,kensipe/dcos,BenWhitehead/dcos,BenWhitehead/dcos,darkonie/dcos,lingmann/dcos,surdy/dcos