Improve error message for duplicated canonical info-hash

[josecelano]

Parent: #521

When you upload a torrent, you can get an error because there is already another torrent with the same canonical info-hash. The error doesn't give you the pre-existing torrent infohash.

The error:

    #[display(fmt = "A torrent with the same canonical infohash already exists in our database.")]
    CanonicalInfoHashAlreadyExists,

should give you the info-hash, so the user can search/navigate to see the pre-existing torrent.

We can do it like we do for other error enums with the thiserror crate.

use thiserror::Error;

#[derive(Debug, Error)]
pub enum ServiceError {
    #[error("internal server error: {0}")]
    InternalServerError(String),
    // other variants...
}