feat: Support Early Validation

[kzcat]

Description of changes:

This PR enhances the error messages displayed when a CloudFormation changeset creation fails due to Early Validation:

https://aws.amazon.com/about-aws/whats-new/2025/11/cloudformation-dev-test-cycle-validation-troubleshooting/

Before

$ rain deploy sample.yaml
error creating changeset: The following hook(s)/validation failed: [AWS::EarlyValidation::ResourceExistenceCheck]. To troubleshoot Early Validation errors, use the DescribeEvents API for detailed failure information.

After

$ ./rain deploy sample.yaml
error creating changeset: changeset creation failed with validation errors:
  - Resource: TestBucket2 (AWS::S3::Bucket), Validation: BUCKET_EMPTINESS_VALIDATION, Reason: The bucket 'test-early-validation-20251218181322-2' is not empty. You must either delete all objects and versions or use the deletion policy to retain it, otherwise the delete operation will fail.
  - Resource: TestBucket1 (AWS::S3::Bucket), Validation: NAME_CONFLICT_VALIDATION, Reason: Resource of type 'AWS::S3::Bucket' with identifier 'test-early-validation-20251218181322-1' already exists.

[r-heimann]

This would be really awesome to have as a feature