### 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_
part_number = part + 1
            upload_file_ = BytesIO(part_data)
            response = aws_client.s3.upload_part(
                Bucket=s3_bucket,
                Key=key_name,
                Body=upload_file_,
                PartNumber=part_number,
                UploadId=upload_id,
            )
            multipart_upload_parts.append({"ETag": response["ETag"], "PartNumber": part_number})
        with pytest.raises(ClientError) as e:
            aws_client.s3.upload_part(
                Bucket=s3_bucket,
                Key=key_name,
                Body=BytesIO(b""),
                PartNumber=-1,
                UploadId=upload_id,
            )
        snapshot.match("upload-part-negative-part-number", e.value.response)
        # test
        with pytest.raises(ClientError) as e:
            aws_client.s3.complete_multipart_upload(
                Bucket=s3_bucket,
                Key=key_name,
                MultipartUpload={"Parts": list(reversed(multipart_upload_parts))},
                UploadId=upload_id,
            )
        snapshot.match("complete-multipart-unordered", e.value.response)
        response = aws_client.s3.complete_multipart_upload(