### 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_
_key = "key-to-delete-trailing-slash"
        # crea
        aws_client.s3.put_(Bucket=s3_bucket, Key=_key, Body=b"123")
        s3_http_client = aws_http_client_factory("s3", signer_factory=SigV4Auth)
        # Endp
        bucket_url = f"{_bucket_url(s3_bucket)}/?delete&x-id=Deletes"
        delete_body = f"""<Delete>
            <>
                <Key>{_key}</Key>
             </>
        </Delete>
        """
        # Post 
        resp = s3_http_client.post(bucket_url, data=delete_body)
        assert resp.status_code == 200, (resp.content, resp.headers)
        resp_dict = xmltodict.parse(resp.content)
        assert "DeleteResult" in resp_dict
        assert resp_dict["DeleteResult"]["Deleted"]["Key"] == _key
    @markers.aws.validated
    @pytest.mark.skipif(reason="Behaviour not implemented yet")
    def test_complete_multipart_parts_checksum(self, s3_bucket, snapshot, aws_client):