bitcoind: sanity check descriptors were imported

[darosior]

At startup, if a watchonly wallet isn't present we'll create and import the descriptors. If one is, we'll just assume the descriptors were successfully imported in it. It would be more robust to sanity check that descriptors were indeed imported:

revaultd/src/bitcoind/poller.rs

Lines 1715 to 1717 in e72146e

	// TODO: sanity check descriptors are imported when migrating to 0.22
	if !PathBuf::from(bitcoind_wallet_path.clone()).exists() {

[darosior]

lol i had a 2 years old issue for this already: #35. Keeping this one as it's more detailed.