refactor(rust): reduce ServiceError (round 1)

[imobachgs]

The Rust part of the project was introduced just as a command-line interface. However, it has turned into a full blown HTTP/API server (with a command-line) and we plan to move more code from Ruby to Rust.

We are learning Rust as we go, so some old decisions might need to be revisited. I have started by trying to simplify the ServiceError enum, which represents an error but includes variants for many different situation (even unrelated ones). We even have an "anyhow" variant!

This first round includes:

• Defining an error for each HTTP client.
• Defining an error for each store.
• Defining an error for the storage, which includes one variant for each store.
• Remove some ServiceError variants.
• Make use of anyhow in more places of the binary.

Once we have put everything into a better place, we can decide to simplify the whole thing if needed. Let's go step by step.

To do

• Test the changes
• Convert errors with a single variant into structs (?)
• Adapt changes from Self-signed certificate support #2270.
• Update the changes file

Follow-up

• There is another whole category errors that are related to D-Bus. Let's move them to their own error and finish the ServiceError clean-up.
• The HTTP client constructors do not need to be async.
• The D-Bus clients are the "special cases". Perhaps we should consider moving from DomainHTTPClient to DomainClient (and DomainClient to DomainDBusClient).

Warning

Do not merge before #2270. We need to fix some conflicts first.

[jreidinger]

using transparent for base looks good to me, I would just probably use Base as enum key instead of HTTP as it is confusing with identical enum vealue from BaseHTTPClientError.

[imobachgs]

This variant actually wraps BaseHTTPClientError which contains HTTP-related errors (get, post, etc.). The idea is that other errors are more domain-specific.

[jreidinger]

does it make sense to have it transparent here? I think additional line can help with seeing which part of store failed

[imobachgs]

Each error already contain a description (e.g., "Error processing storage settings: {0}").