Fix #223: Allow adding a custom validation error at a given array index

[wowlusitong]

Reasons for making this change

For #223

Checklist

• I'm updating documentation
  • I've checked the rendering of the Markdown text I've added
  • If I'm adding a new section, I've updated the Table of Content
• I'm adding or updating code
  • I've added and/or updated tests
  • I've updated docs if needed
  • I've run npm run cs-format on my branch to conform my code to prettier coding style
• I'm adding a new feature
  • I've updated the playground with an example use of the feature

[n1k0]

Looks very good, thanks!

[n1k0]

Released in v0.45.0.