### 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_
snapshot.add_transformers_list(
            [
                snapshot.transform.key_value("HostId"),
                snapshot.transform.key_value("RequestId"),
            ]
        )
        _key = "test-presigned-post-key-storage-class"
        presigned_request = aws_client.s3.generate_presigned_post(
            Bucket=s3_bucket,
            Key=_key,
            ExpiresIn=60,
            Fields={
                "x-amz-storage-class": StorageClass.STANDARD_IA,
            },
            Conditions=[
                {"bucket": s3_bucket},
                ["eq", "$x-amz-storage-class", StorageClass.STANDARD_IA],
            ],
        )
        # Pos
        response = .post(
            presigned_request["url"],
            data=presigned_request["fields"],
            files={"file": "test-body-storage-class"},
            verify=False,
        )
        assert response.status_code == 204
        head_ = aws_client.s3.head_(Bucket=s3_bucket, Key=_key)
        snapshot.match("head-", head_)