### 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_
"Bucket": s3_bucket,
                "Key": _key,
                "ResponseCacheControl": "max-age=74",
                "ResponseContentDisposition": 'attachment; filename="foo.jpg"',
                "ResponseContentEncoding": "identity",
                "ResponseContentLanguage": "de-DE",
                "ResponseContentType": "image/jpeg",
                "ResponseExpires": expiry_date,
            },
        )
        response = .get(url, verify=False)
        assert response.status_code == 200
        headers = response.headers
        assert headers["cache-control"] == "max-age=74"
        assert headers["content-disposition"] == 'attachment; filename="foo.jpg"'
        assert headers["content-encoding"] == "identity"
        assert headers["content-language"] == "de-DE"
        assert headers["content-type"] == "image/jpeg"
        # Note
        possible_date_formats = ["2015-10-21T07:28:00Z", expiry_date]
        assert headers["expires"] in possible_date_formats
    @markers.aws.validated
    @markers.snapshot.skip_snapshot_verify(
        condition=is_v2_provider,
        paths=["$..ServerSideEncryption"],
    )
    def test_s3_copy_md5(self, s3_bucket, snapshot, monkeypatch, aws_client):
        if not is_aws_cloud():
            monkeypatch.setattr(config, "S3_SKIP_SIGNATURE_VALIDATION", False)
        src_key = "src"