### Is there an existing issue for this?

- [X] I have searched the existing issues

### Current Behavior

- Localstack fails with HTTP 500, error code "InternalError", message "not enough values to unpack (expected 2, got 1)" when `CopySource` is an invalid string for `copy_object` API.

### Expected Behavior

- For the same API `copy_object`, the cloud fails with HTTP 400, error code "InvalidArgument".
- Localstack should handle invalid `CopySource` i.e., it is a dictionary with one value or an invalid string not adhering to "{bucket}/{key}" format.

### How are you starting LocalStack?

With a `docker run` command

### Steps To Reproduce

#### How are you starting localstack (e.g., `bin/localstack` command, arguments, or `docker-compose.yml`)

    docker run localstack/localstack

#### Client commands (e.g., AWS SDK code snippet, or sequence of "awslocal" commands)

    botoclient.copy_object(CopySource="Invalid_random_string", Bucket="test_bucket", Key=key) 


 test_bucket is a valid bucket.  

### Environment

```markdown
- OS: Ubuntu 22.04.2 LTS
- LocalStack: 2.1.0
- boto3: 1.28.6
```


### Anything else?

_No response_
aws_client.s3.put_(Bucket=s3_bucket, Key=src_key, Body="something")
        # copy 
        dest_key = "dest"
        response = aws_client.s3.copy_(
            Bucket=s3_bucket,
            CopySource={"Bucket": s3_bucket, "Key": src_key},
            Key=dest_key,
        )
        snapshot.match("copy-obj", response)
        presigned_client = _s3_client_pre_signed_client(
            Config(signature_version="s3v4", s3={"payload_signing_enabled": True}),
            endpoint_url=_endpoint_url(),
        )
        # Crea
        dest_key2 = "dest"
        url = presigned_client.generate_presigned_url(
            "copy_",
            Params={
                "Bucket": s3_bucket,
                "CopySource": {"Bucket": s3_bucket, "Key": src_key},
                "Key": dest_key2,
            },
        )
        request_response = .put(
            url, headers={"x-amz-copy-source": f"{s3_bucket}/{src_key}"}, verify=False
        )
        assert request_response.status_code == 200
    @markers.aws.only_localstack
    @pytest.mark.parametrize("case_sensitive_headers", [True, False])
    def test_s3_get_response_case_sensitive_headers(