### 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_
endpoint_url=_endpoint_url(),
        )
        presigned_request = presigned_client.generate_presigned_post(
            Bucket=s3_bucket, Key=_key, ExpiresIn=60
        )
        # modi
        original_policy = presigned_request["fields"]["policy"]
        presigned_request["fields"]["policy"] = original_policy[:-2]
        response = .post(
            presigned_request["url"],
            data=presigned_request["fields"],
            files={"file": "file content"},
            verify=False,
        )
        # the 
        exception = xmltodict.parse(response.content)
        exception["StatusCode"] = response.status_code
        snapshot.match("exception-policy", exception)
        # asse
        signature_field = "signature" if signature_version == "s3" else "x-amz-signature"
        assert (
            exception["Error"]["SignatureProvided"] == presigned_request["fields"][signature_field]
        )
        assert exception["Error"]["StringToSign"] == presigned_request["fields"]["policy"]
    @markers.aws.validated
    @pytest.mark.parametrize(
        "signature_version",
        ["s3", "s3v4"],
    )
    def test_post_request_missing_signature(