Wallet::get_address should return an Result

[LLFourn]

Since the persistent backend is fallible. There are expect("TODO") in there now which seem to have slipped through the cracks. It would be good to differentiate between those calls that are fallible and those that are infallible.
I think AddressIndex should go in favor of different methods that return different addresses since the error cases seem to be different.

I think with an infallible persistence backend we can have new_address but if it's fallible then it should be try_new_address and return a Result.

cc @notmandatory. IIRC you were thinking about how to redesign this.

[nondiremanuel]

Is this part of 1.0 version or should I assign it to maintenance?

[LLFourn]

Thanks i put 1.0.0 on now.

[nondiremanuel]

@LLFourn FYI I removed the 1.0.0 milestone both from this and from the #1005 issue since we are using the Project "BDK 1.0" for issues / PRs related in general to 1.0 version. We use the milestones (alpha.1, alpha.2 ...) to signal the "sub-version" in which the specific ticket will eventually be delivered in

[LLFourn]

This should be fixed before 1.0.0-alpha.1. at least the expect("TODO") bits. A redesign can come in a further release so we just need a small quick fix for now.

[notmandatory]

Removed from alpha.1, can see if able to get enough review for alpha.2.