Ansible Ignore errors in tasks and fail at end of the playbook if any tasks had errors

Use Fail module.

1. Use ignore_errors with every task that you need to ignore in case of errors.
2. Set a flag (say, result = false) whenever there is a failure in any task execution
3. At the end of the playbook, check if flag is set, and depending on that, fail the execution

- fail: msg="The execution has failed because of errors."
  when: flag == "failed"

Update:

Use register to store the result of a task like you have shown in your example. Then, use a task like this:

- name: Set flag
  set_fact: flag = failed
  when: "'FAILED' in command_result.stderr"