How do I run ansible on one host at a time and break on a failure
Your playbook will stop when a failure occurs and you're using serial: 1 according to the documentation.


  By default, Ansible will continue executing actions as long as there are hosts in the group that have not yet failed. 


That said there seems to be some confusion in the community over the default behavior, and it seems to have changed--or been buggy--somewhere between 1.8 and 2.1.

So, if serial: 1 doesn't suffice, use this additional setting:

max_failure_percentage: 0


  In some situations, such as with the rolling updates described above, it may be desirable to abort the play when a certain threshold of failures have been reached. To achieve this, as of version 1.3 you can set a maximum failure percentage...


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As for retrying your playbook, you should be seeing a failure message like this:

to retry, use: --limit @/home/user/site.retry

Use that --limit flag and on your next execution of ansible-playbook and it will continue from where it failed.

Retry files will be created unless you've set retry_files_enabled = False in your configuration.

Alternatively, --start-at-task may also work.

Sources:

https://github.com/ansible/ansible/issues/1663

https://github.com/ansible/ansible/issues/16241

http://docs.ansible.com/ansible/playbooks_delegation.html#rolling-update-batch-size

http://docs.ansible.com/ansible/playbooks_delegation.html#maximum-failure-percentage

http://docs.ansible.com/ansible/intro_configuration.html#retry-files-enabled

http://docs.ansible.com/ansible/playbooks_startnstep.html#start-at-task