feat: new method for choice fail

[antazoey]

Refactors so the failing happens in a new public method.

• fixes Allow customizing fail message for invalid choice in click.types.Choice #2621

Ensure each step in CONTRIBUTING.rst is complete by adding an "x" to
each box below.

If only docs were changed, these aren't relevant and can be removed.
-->

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.

[AndreasBackx]

This seems like a fine choice. Could you follow through with the rest of the checklist like documentation and tests?

[antazoey]

I did my best to complete the checkboxes. Notes:

• I wasn't sure where to put the test as there were no other Choice-based tests in types module however the places testing choice were more integraion-y and less about the helper messages; so the test I added stands out like a lone wolf.
• Nothing is really "changing" so I opted for not putting .. versionchanged:: anywhere
• I didn't see any docs/ relevant to making custom choice types so I didn't any the top-level use-case there.
• I did add a methoc doc though just for the sake of the checkbox, however the other helper fail fn doesn't have a method doc, so this stands out.

[AndreasBackx]

@antazoey thanks for getting back so quickly and my apologies for taking another week myself. I've wrapped it up and merged it, thank you for this fix!