Use anyhow to add context to errors

[leplatrem]

Mike recommends us to use anyhow:

For what it's worth, I've had a lot of success combining both thiserror and anyhow. The strings here aren't intended to be machine-readable. Using anyhow lets you preserver more context while keeping the free form nature that strings provide.

Probably not worth changing in this PR, but maybe something to consider for 1.0?

Source #133 (comment)

Example:

        let client = redis::Client::open(settings.redis.url.clone())
            .context("Setting up Redis client")
            .map_err(SetupError::Network)?;