add thiserror to sozu-command-lib

[Keksoj]

Using anyhow in Sōzu has proven valuable to concatenate context accross function, and display the whole context all in one line for optimal readability.

This readability is only an advantage to humans, though.

We aim to make errors more usable for machines, with proper error enums to be pattern-matched and so on.

[Keksoj]

This is to solve this issue:

• Use the thiserror crate for better error management #910

[FlorentinDUBOIS]

Hey 👋 , I started the review, but I do not want to create a lot of comments.
Could we avoid to

• Create items on error that ends with Error, it is redundant (aka ChannerError::FooError)
• Have generic message error like IoError(std::io::Error) and with a message like read error
• Create redendundant entries like (ClusterNotFound, ListenerNotFound and so on..), but instead have an error like NotFound { kind: AnEnum, id: String }

[Keksoj]

thanks @FlorentinDUBOIS , I removed all ...Error endings in variants, it's more legible 👍 and I merged redundant error types in NotFound or NoMetrics variants.

[FlorentinDUBOIS]

Nice works ! There are some stuffs to do, but we are quite close to merge it.

[Keksoj]

this is ready for review BTW