Add error_key field option. Closes #318 (v3)

[mattbrictson]

This is a cherry-pick of #333 onto master instead of v4.

[bootstrap-ruby-bot]

	1 Warning
⚠️	Please update CHANGELOG.md with a description of your changes. If this PR is not a user-facing change (e.g. just refactoring), you can disregard this.

Here's an example of a CHANGELOG.md entry (place it immediately under the * Your contribution here! line):

* [#334](https://github.com/bootstrap-ruby/rails-bootstrap-forms/pull/334): Add error_key field option. Closes #318 (v3) - [@mattbrictson](https://github.com/mattbrictson).

Generated by 🚫 Danger

[sebfie]

Hello, I think it's a right pr to merge :)

[donv]

@mattbrictson This looks like a good change. Can you rebase onto main?

[donv]

The demo app was moved from test/dummy to demo so it is a bit difficult to rebase the change.

@mattbrictson Do you still want this change?.

[mattbrictson]

I have forgotten the context or need that motivated me to open this PR. It is safe to say I no longer need it. I will close.