Merge bitcoindevkit/bdk#1256: cherry-pick feat(wallet)!: add `NonEmptyDatabase` varia...

a1d34afa2455d93d23a36fd78d94aa79c7e493d1 feat(wallet)!: add `NonEmptyDatabase` variant to `NewError` (志宇)

Pull request description:

  ### Description

  `NewError` is the error type when constructing a wallet with `Wallet::new`. We want this to return an error when the database already contains data (in which case, the caller should use `load` or `new_or_load`).

  ### Notes to the reviewers

  This is cherry-picked from #1172 so that we can add it to the alpha.3 release.

  ### Changelog notice

  Change
  - Return `NonEmptyDatabase` error when constructing a wallet with `Wallet::new` if the file already contains data (in which case, the caller should use `load` or `new_or_load`).

  ### Checklists

  #### All Submissions:

  * [x] I've signed all my commits
  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)
  * [x] I ran `cargo fmt` and `cargo clippy` before committing

  #### New Features:

  * [x] I've added tests for the new feature
  * [x] I've added docs for the new feature

ACKs for top commit:
  evanlinjin:
    ACK a1d34afa2455d93d23a36fd78d94aa79c7e493d1

Tree-SHA512: 7c20171fa3d7dee5b1ac24f8a808781dbb0be0034951005e1e87acdf023123c01161e225b47b6d4484865889778c39549a3780f641227ddc0f84d1577d69f40a