Java Mockito: How to test when a private exception is thrown? (Not a private method)

I have a test that throws HttpException from a certain library (com.googlecode.jsonrpc4j). I have written a test like so:

Exception exception = assertThrows(HttpException.class,
        () -> this.myApi.getInformation(date1, date2)
    );

For this, I have imported the HttpException class like so:

import com.googlecode.jsonrpc4j.HttpException;

However, doing so gives me an error message:

'com.googlecode.jsonrpc4j.HttpException' is not public in 'com.googlecode.jsonrpc4j'. 
Cannot be accessed from outside package

So, even though this exception is thrown in certain conditions, I am unable to test that this exception will be thrown under those conditions. Is there any way that I can test this?

My workaround, since the private HttpException extends RuntimeException, I can test it by asserting that RuntimeException is thrown. However, is there a better way to test this?