Switch to Snafu for error handling

[sandhose]

We're not always doing a good job at reporting errors.

A few factors for this:

• we rely on tracing::instrument(err) for logging when some function fails. This usually means that when an error bubbles up, it gets logged multiple times, with many times a lot of the tracing context in it
• we don't consistently report them to sentry. In a lot of cases, we have a RouteError that calls sentry::capture_error, but we're missing that in multiple places
• we use a mix of thiserror and anyhow, not very consistently. The fact that anyhow::Error doesn't implement std::error::Error is annoying
• we often want to attach random string context to errors, which is annoying to do. In syn2mas we started using thiserror-ext exactly for that
• we don't collect enough infos on errors. It would be nice to capture the code location where it was emitted, maybe the span context as well

Snafu looks nice to do all this:

• it covers both the 'error derive macro' like thiserror and the 'whatever' error like anyhow
• it automatically creates helper to add context on results, bundling the source error
• when gathering context, it can automatically gather stuff like the backtrace, the code location or the span context
• it's overall more opinionated, in a good way?
• it adopts the new error 'provide' API. We could use that to try to define well-known error codes so that we can translate those to users?

We can try to gradually adopt it, as it works well with any error that implements std::error::Error anyway (which is the case for most of our code)

Related to #1488