Show user validation errors after generating event/metric definitions

What does this MR do and why?

• Changes: Validates generated event & metric definitions when generated with scripts/internal_events/cli.rb, displaying any errors to the user
• Motivation: By displaying the errors instead of blocking generation completely, we make sure users aren't blocked or stuck during generation, but we still offer more transparent information

Related issue: #441143 (closed)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

1. Run scripts/internal_events/cli.rb
2. Select New Event
3. Enter a description
4. Enter an event name
5. Select project+namespace+user
6. Leave MR blank
7. Select a group
8. Select free+premium+ultimate
9. Create the file
10. See the error message
11. Select Create Metric
12. Select the first metric option
13. Enter two descriptions
14. Select Copy & continue
15. Create the files
16. See the error message