Sign then Encrypt

Ok. Hopefully this addresses all the past feedback. I tried to simplify the PR and still get what we need in. This one is also a prerequisite for #840 as mentioned below. @Renelvon @gubatron take a look.
Sorry, the web interface wasn't helpful here and I could have written one or two more lines.
I was asking why do we need to use a try around self.encrypt. In what cases can it throw an exception?