Replace err-context with snafu

[MarkusPettersson98]

This PR replaces the use of err-context for error message mangling with the well-maintained snafu crate. err-context has not seen updates in 6+ years. It is probably done, but it is a bit of an obscure relic from the past. snafu does what it does without adding to much bloat while being widely used & well-maintained.

This PR changes the format of some internal errors. If the output format of some internal error is important, we should add snapshot tests to cover that.

I base this PR on top of #77 due to https://github.com/mullvad/udp-over-tcp/actions/runs/17802660071/job/50606293583. If accepted, it should be merged to main.

This is the general format of domain specific errors generated by snafu

Error: <our specific domain error happened>

Caused by this error:
1: <source error, e.g. "No such file or directory (os error 2)">

Edit: If formatting these internal errors is not important at all, we might as well drop both err-context and snafu.

---

This change is