### 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_
# get 
        _key = "key-${filename}"
        presigned_request = aws_client.s3.generate_presigned_post(
            Bucket=s3_bucket,
            Key=_key,
            Fields={"success_action_status": "201"},
            Conditions=[{"bucket": s3_bucket}, ["eq", "$success_action_status", "201"]],
            ExpiresIn=60,
        )
        files = {"file": ("my-file", body)}
        response = .post(
            presigned_request["url"],
            data=presigned_request["fields"],
            files=files,
            verify=False,
        )
        assert response.status_code == 201
        json_response = xmltodict.parse(response.content)
        assert "PostResponse" in json_response
        json_response = json_response["PostResponse"]
        location = f"{_bucket_url_vhost(s3_bucket, region_name)}/key-my-file"
        etag = '"43281e21fce675ac3bcb3524b38ca4ed"'
        assert response.headers["ETag"] == etag
        assert response.headers["Location"] == location
        assert json_response["Location"] == location
        assert json_response["Bucket"] == s3_bucket
        assert json_response["Key"] == "key-my-file"
        assert json_response["ETag"] == etag
    @markers.aws.validated
    def test_s3_presigned_post_success_action_redirect(self, s3_bucket, aws_client):