Make ErrorReport not be Sized so we can return Box<dyn ErrorReport> for Arti errors

It'd be nice to have all Tor-related errors be represented as Box<dyn ErrorReport> so we can call the report() method on them for a nice, pretty-printed error.

This way, other application developers could possibly use the report output and display it in different ways, as well as be able to represent Arti-related errors conveniently.