### 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_
endpoint_url=s3_endpoint_path_s,
        )
        upload_id = client.create_multipart_upload(
            Bucket=bucket_name,
            Key=_key,
        )["UploadId"]
        data = to_bytes("hello this is a upload test")
        upload_file_ = BytesIO(data)
        signed_url = _generate_presigned_url(
            client,
            {
                "Bucket": bucket_name,
                "Key": _key,
                "UploadId": upload_id,
                "PartNumber": 1,
            },
            expires=4,
            client_method="upload_part",
        )
        response = .put(signed_url, data=upload_file_)
        assert response.status_code == 200
        multipart_upload_parts = [{"ETag": response.headers["ETag"], "PartNumber": 1}]
        response = client.complete_multipart_upload(
            Bucket=bucket_name,
            Key=_key,
            MultipartUpload={"Parts": multipart_upload_parts},
            UploadId=upload_id,
        )
        assert response["ResponseMetadata"]["HTTPStatusCode"] == 200
        simple_params = {"Bucket": bucket_name, "Key": _key}