Add `DbClosedError`

[GodTamIt]

This adds a new exception type for when the instance is already closed.

It also cleans up common access patterns to be more succinct:

let db = self.get_db()?;

instead of

if let Some(db) = &self.db {
   unimplemented!("Some code here")
} else {
    Err(...)
}

[Congyuwang]

Thanks for contributing. This is very nice!