### 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_name = "test-upload-part-exc"
        response = aws_client.s3.create_multipart_upload(Bucket=s3_bucket, Key=key_name)
        upload_id = response["UploadId"]
        parts = []
        for i in range(1, 3):
            upload_part = aws_client.s3.upload_part(
                Bucket=s3_bucket,
                Key=key_name,
                Body=BytesIO(b"data"),
                PartNumber=i,
                UploadId=upload_id,
            )
            parts.append({"ETag": upload_part["ETag"], "PartNumber": i})
            snapshot.match(f"upload-part{i}", upload_part)
        with pytest.raises(ClientError) as e:
            aws_client.s3.complete_multipart_upload(
                Bucket=s3_bucket, Key=key_name, UploadId=upload_id
            )
        snapshot.match("complete-exc-no-parts", e.value.response)
        with pytest.raises(ClientError) as e:
            aws_client.s3.complete_multipart_upload(
                Bucket=s3_bucket, Key=key_name, UploadId=upload_id, MultipartUpload={"Parts": parts}
            )
        snapshot.match("complete-exc-too-small", e.value.response)
    @pytest.mark.skipif(condition=LEGACY_V2_S3_PROVIDER, reason="not implemented in moto")
    @markers.aws.validated
    def test_multipart_complete_multipart_wrong_part(self, s3_bucket, snapshot, aws_client):