### 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_
# snap
        snapshot.match("create-kms-key", kms_key)
        # test 
        if not is_aws_cloud():
            key_name = "test-sse-validate-kms-key-no-check"
            response = client.put_(
                Bucket=bucket_name,
                Key=key_name,
                Body=data,
                ServerSideEncryption="aws:kms",
                SSEKMSKeyId="fake-key-id",
            )
            assert response["ResponseMetadata"]["HTTPStatusCode"] == 200
            response = client.create_multipart_upload(
                Bucket=bucket_name,
                Key="multipart-test-sse-validate-kms-key-no-check",
                ServerSideEncryption="aws:kms",
                SSEKMSKeyId="fake-key-id",
            )
            assert response["ResponseMetadata"]["HTTPStatusCode"] == 200
            response = client.copy_(
                Bucket=bucket_name,
                Key="copy-test-sse-validate-kms-key-no-check",
                CopySource={"Bucket": bucket_name, "Key": key_name},
                ServerSideEncryption="aws:kms",
                SSEKMSKeyId="fake-key-id",
            )
            assert response["ResponseMetadata"]["HTTPStatusCode"] == 200
        key_name = "test-sse-validate-kms-key"
        fake_key_uuid = "134f2428-cec1-4b25-a1ae-9048164dba47"