EC2 Key Pair creation fails with unhelpful error

### Name of the resource

Other

### Resource name

AWS::EC2::KeyPair

### Description

This was originally reported in CDK in [aws/aws-cdk:25929](https://github.com/aws/aws-cdk/issues/25929). When the value of `PublicKeyMaterial` is not well formatted, the creation of `AWS::EC2::KeyPair` fails with unhelpful error like:

```
Resource handler returned message: "null" (RequestToken: c6817ee0-0c65-b552-f35f-336715c567ce, HandlerErrorCode: InvalidRequest)
```

CloudFormation snippet to reproduce:

```yaml
Resources:
MyKeyPair:
Type: AWS::EC2::KeyPair
Properties:
KeyName: keyName
PublicKeyMaterial: somethingInvalid
```

It would be great if CloudFormation propagated the error from the back-end service.

### Other Details

_No response_

Start with the provided CloudFormation YAML using AWS::EC2::KeyPair and the invalid PublicKeyMaterial value, then compare the behavior with the linked CDK report. Confirm the backend response and determine whether CloudFormation can expose it instead of returning "null". Done means the same invalid input produces a useful propagated error.

Written by the indexing model from the issue text.