### 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_transformer(
            [
                snapshot.transform.key_value("Bucket", reference_replacement=False),
                snapshot.transform.key_value("Location"),
                snapshot.transform.key_value("UploadId"),
                snapshot.transform.key_value("DisplayName", reference_replacement=False),
                snapshot.transform.key_value("ID", reference_replacement=False),
            ]
        )
        key_name = "test-multipart-checksum"
        response = aws_client.s3.create_multipart_upload(
            Bucket=s3_bucket, Key=key_name, ChecksumAlgorithm="SHA256"
        )
        snapshot.match("create-mpu-checksum", response)
        upload_id = response["UploadId"]
        
        part_data = "a" * (5_242_880 + 1)