Better exception handling

Changed in two places:

1. Was rescuing Exception in Heroku::Nav::Header, but I made it more specific by rescuing Timeout::Error and StandardError. This will allow it to correctly handle other exceptions like interrupts.

2. Was rescuing StandardError in Heroku::Nav::Provider, which was not catching Timeout::Error. Made the same change here. This will allow it to allow it to gracefully handle timeout exceptions.

Thanks!
--Brandon