upgrade and improve error handling

[jcrowgey]

I've got a rudimentary custom error type which (on current rust) generates deprecation warnings at compile time:

warning: trait objects without an explicit `dyn` are deprecated
  --> src/errors.rs:17:32
   |
17 |     fn cause(&self) -> Option<&error::Error> {
   |                                ^^^^^^^^^^^^ help: use `dyn`: `dyn error::Error`
   |
   = note: #[warn(bare_trait_objects)] on by default

warning: trait objects without an explicit `dyn` are deprecated
  --> src/question.rs:44:32
   |
44 |     fn cause(&self) -> Option<&error::Error> {
   |                                ^^^^^^^^^^^^ help: use `dyn`: `dyn error::Error`

I need to grok proper error handling in Rust and upgrade lud accordingly.