docs: Add error handling after container purging

[sryoya]

Related issue

none.
As this has just minor changes, I created this PR without prior discussion.

Proposed changes

Adds error handling after pool.Purge in example codes README.md is already doing.
It may be trivial, but I think it's worth adding so that users don't forget to add this handling when they are creating their own codes with referring these examples.

Checklist

• I have read the contributing guidelines
• I have read the security policy
• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security vulnerability, I
  confirm that I got green light (please contact
  security@ory.sh) from the maintainers to push
  the changes.
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation within the code base (if appropriate)

Further comments

[CLAassistant]

All committers have signed the CLA.

[aeneasr]

Awesome 🎉

Thank you for your contribution!