Support rendering Exceptions with previous Errors
Support rendering Exceptions with previous Errors

If an Exception is thrown with an Error set as it's previous then we
need to render this correctly in the error handler. This simply requires
removing the type hint, however we can't do that for BC, so I've created
the method `renderHtmlExceptionOrError` which is now used.

Fixes #1943

iinux/Slim,iinux/Slim,akrabat/Slim,iinux/Slim,juliangut/Slim,slimphp/Slim,foxyantho/Slim,Sam-Burns/Slim