### 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_
)
        # put 
        response = .post(
            presigned_request["url"],
            data=presigned_request["fields"],
            files={"file": "test-body-tagging"},
            verify=False,
        )
        if tagging == "not-xml":
            assert response.status_code == 400
            snapshot.match("tagging-error", xmltodict.parse(response.content))
            with pytest.raises(ClientError) as e:
                aws_client.s3.get__tagging(Bucket=s3_bucket, Key=_key)
            e.match("NoSuchKey")
        else:
            assert response.status_code == 204
            tagging = aws_client.s3.get__tagging(Bucket=s3_bucket, Key=_key)
            snapshot.match("get-tagging", tagging)
    @markers.aws.validated
    @markers.snapshot.skip_snapshot_verify(
        condition=is_v2_provider,
        paths=["$..ServerSideEncryption"],
    )
    @markers.snapshot.skip_snapshot_verify(
        paths=[
            "$..ContentLength",
            "$..ETag",
        ],  # FIXME: in CI, it fails sporadically and the form is empty
    )
    def test_post__with_metadata(self, s3_bucket, aws_client, snapshot):