### 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=presigned_request["fields"],
            files={"file": "file content"},
            verify=False,
        )
        exception = xmltodict.parse(response.content)
        exception["StatusCode"] = response.status_code
        snapshot.match("exception-missing-fields", exception)
        # pop 
        presigned_request["fields"] = {
            k: v for k, v in presigned_request["fields"].items() if k in ("key", "policy")
        }
        response = .post(
            presigned_request["url"],
            data=presigned_request["fields"],
            files={"file": "file content"},
            verify=False,
        )
        exception = xmltodict.parse(response.content)
        exception["StatusCode"] = response.status_code
        snapshot.match("exception-no-sig-related-fields", exception)
    @markers.aws.validated
    @pytest.mark.xfail(
        reason="sporadically failing in CI: presigned-post does not set the body, and then etag is wrong",
    )
    def test_s3_presigned_post_success_action_status_201_response(
        self, s3_bucket, aws_client, region_name
    ):
        # a se
        # see 
        body = "something body"