### 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_
handler_file=os.path.join(
                os.path.dirname(__file__),
                "../lambda_",
                "functions",
                "lambda_triggered_by_sqs_download_s3_file.py",
            ),
            func_name=function_name,
            role=lambda_su_role,
            runtime=Runtime.python3_9,
            envvars=dict(
                {
                    "BUCKET_NAME": bucket_name,
                    "OBJECT_NAME": key,
                    "LOCAL_FILE_NAME": "/tmp/" + key,
                }
            ),
        )
        aws_client.lambda_.invoke(FunctionName=function_name, InvocationType="Event")
        # TODO 
        retry(
            check_expected_lambda_log_events_length,
            retries=10,
            sleep=1,
            function_name=function_name,
            regex_filter="success",
            expected_length=1,
            logs_client=aws_client.logs,
        )
    @markers.aws.validated
    def test_precondition_failed_error(self, s3_create_bucket, snapshot, aws_client):