Update example for Koa

[ad-m]

I make small suggestion to koa example to make it self-contained without need to analyze other examples.

• Added error handler
• Removed module.exports and return
• Use 'const' for variable declaration

[github-actions[bot]]

:tada: This PR is included in version 3.2.5 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot :package::rocket: