PDL: let ‘find_binary’ helper return a ‘Result’

This allows us to show what we searched for and where. It makes the
error messages much more useful when you run ‘cargo test’ without
having the necessary binaries in your ‘$PATH’.

Test: ‘cargo test’ without having run ‘m pdl’
Change-Id: I98b3cc3c9d10fa930bc449faddc5c6186583ef54