Add recaptcha_failure_reason for enchanced error reporting

[holli]

This update introduces the recaptcha_failure_reason method to provide developers with detailed information on common verification failures. This enhancement aims to improve debugging and error handling when integrating to applications.

I had annoying moments debugging errors when using the gem. Gem itself worked nicely but the error messages needed some manual work. E.g. when custom javascripts set the request parameter correctly but with wrong name, the gem didn't give any errors it just failed. I wanted to have simpler error messages. This also helps when logging the results as now I have single method to return some information.

Key Changes:

• Added recaptcha_failure_reason method in controller_methods.rb to return detailed failure reasons.
• Updated verify_recaptcha method to set @_recaptcha_failure_reason with specific error messages based on the verification outcome.
• Modified verify_recaptcha! to raise VerifyError with an informative error message.
• Updated tests in test/verify_test.rb to cover new failure scenarios and ensure recaptcha_failure_reason provides accurate information.

Pre-Merge Checklist

• CHANGELOG.md updated with short summary for user facing changes

[grosser]

this is super nice, thx!

[grosser]

5.18.0