### 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_
data = "upload-part-1" * 5
        data = to_bytes(data)
        upload_file_ = BytesIO(data)
        response = aws_client.s3.upload_part(
            Bucket=s3_bucket,
            Key=key_name,
            Body=upload_file_,
            PartNumber=1,
            UploadId=upload_id,
        )
        snapshot.match("upload-part", response)
        list_part = aws_client.s3.list_parts(Bucket=s3_bucket, Key=key_name, UploadId=upload_id)
        snapshot.match("list-part-after-upload", list_part)
        list_multipart_uploads = aws_client.s3.list_multipart_uploads(Bucket=s3_bucket)
        snapshot.match("list-all-uploads-after", list_multipart_uploads)
        multipart_upload_parts = [{"ETag": response["ETag"], "PartNumber": 1}]
        response = aws_client.s3.complete_multipart_upload(
            Bucket=s3_bucket,
            Key=key_name,
            MultipartUpload={"Parts": multipart_upload_parts},
            UploadId=upload_id,
        )
        snapshot.match("complete-multipart", response)
        with pytest.raises(ClientError) as e:
            aws_client.s3.list_parts(Bucket=s3_bucket, Key=key_name, UploadId=upload_id)
        snapshot.match("list-part-after-complete-exc", e.value.response)
        list_multipart_uploads = aws_client.s3.list_multipart_uploads(Bucket=s3_bucket)
        snapshot.match("list-all-uploads-completed", list_multipart_uploads)
    @markers.aws.validated
    def test_multipart_no_such_upload(self, s3_bucket, snapshot, aws_client):