Issue #531: Fix missing impl for TransactionError

[alecsmrekar]

This PR addressed issue #531.

The original bug report provided a minimum reproducible code, but I think we're able to shrink that even further to:

async fn test_case(user: &mut GooseUser) -> TransactionResult {
    let goose = user
        .get("my/url")
        .await?;
    let _ = goose.response?;
    Ok(())
}

Trying to compile a loadtest with this code should give the following error:

the trait `From<reqwest::error::Error>` is not implemented for `Box<TransactionError>`

With this PR, it should compile cleanly.

[jeremyandrews]

Thanks! Closes #531