If you are not familiar with modules, check out Ansible - Getting Started with Modules.

There are a number of ways to do something when a task returns an error.

• Use fail to end a play and return "failed" (this article)
• Use the --check command line flag to run the playbook in a dry run mode
• Use check_mode to determine if any fatal errors would be returned
• Use failed_when to continue based on the task return code
• Use ignore_errors to ignore errors returned by a task
• Use meta: clear_host_errors to clear errors so that subsequent tasks are run
• Use meta: end_play to end a play for all hosts
• Use meta: end_host to end a play for certain hosts and return "skipped"
• Use block rescue always to run tasks even after a fatal error is returned
• Use stat to determine if /tmp/foo.txt exists and the when parameter to skip the lineinfile module if /tmp/foo.txt does not exist.
• Use stat to determine if /tmp/foo.txt exists and then use the file module and the when parameter to create /tmp/foo.txt if the file does not exist

The fail module is used to end a play for a host when a certain condition is met. In this example, the playbook will fail when the foo variable is undefined.

---
- hosts: localhost
  tasks:
  - name: fail when the 'foo' variable is undefined
    fail: 
      msg: the 'foo' variable is undefined
    when: foo is undefined
...

It's important to recognize that when the when condition in the fail module evaluates to true:

• The play will be ended for the host meaning that the subsequent tasks in the playbook will not be executed on the host
• The task to return fatal and the PLAY RECAP with have failed=1.

meta: end_host can be used if you want the PLAY RECAP to have "skipped" instead of "failed".

TASK [fail when the 'foo' variable is undefined]
fatal: [server1.example.com]: FAILED! => {"changed": false, "msg": "The 'foo' variable is undefined"}

PLAY RECAP *********************************************************************
server1.example.com    : ok=0   changed=0    unreachable=0    failed=1    skipped=0    rescued=0    ignored=0