Remove deprecation warnings

Context: https://doc.rust-lang.org/reference/types/trait-object.html

Before:

warning: trait objects without an explicit `dyn` are deprecated
  --> src/simple_api/mod.rs:60:32
   |
60 |     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/simple_api/mod.rs:65:39
   |
65 | pub type YDBResult<T> = Result<T, Box<Error>>;
   |                                       ^^^^^ help: use `dyn`: `dyn Error`
... snip ...

After: no warnings