Add std::error::Error impls for error types

[ronnodas]

The trait is meant for error types. This also makes it more convenient for users wrapping biblatex's error types into their own errors and using the thiserror crate to derive error traits.

[jonaspleyer]

I have just opened another PR which basically does the same. You got there first. LGTM

[anwaralameddin]

Hello, I am running into the same issue when wrapping biblatex error types with a structure implementing the thiserror::Error trait. Currently, using the main branch, the following code errors

use thiserror::Error;

#[derive(Debug, Error)]
pub enum MyError {
    #[error("TypeError: {0}")]
    TypeError (#[from] TypeError),
}

Temporally, I am using the following instead

use thiserror::Error;

#[derive(Debug, Error)]
pub enum MyError {
    #[error("TypeError: {0}")]
    TypeError (TypeError),
}

impl From<TypeError> for MyError {
    fn from(err: TypeError) -> Self {
        BibError::TypeError(err)
    }
}

It would be helpful if this PR could be progressed to avoid the need for this, especially since the issue applies also to RetrievalError and ParseError.

Also, in line with the OP, in general, error types are expected to implement the std::error::Error trait.

[PgBiel]

Thanks!

[PgBiel]

Thank you!