Drop panic if `rust-bitcoin` adds a new `Network`

Drop panic if `rust-bitcoin` adds a new `Network`

`rust-bitcoin` 0.30 added `#[non_exhaustive]` to the `Network`
enum, allowing them to "add support" for a new network type without
a major version change in the future. When upgrading, we added a
simple `unreachable` for the general match arm, which would break
in a minor version change of `rust-bitcoin`.

While it seems [possible rust-bitcoin will change
this](https://github.com/rust-bitcoin/rust-bitcoin/issues/2225),
we still shouldn't ba panicking, which we drop here in favor of a
`debug_assert`ion, and a default value.