Merge bitcoindevkit/bdk#1008: Backporting "Fix policy condition calculation" onto...

9cffaad71f3d090196a3e35e84ad714118ea2846 Fix build errors (junderw)
3ccdb84523997d3a9582b1ebd2cf2fe373ddaa55 Fix policy condition calculation When constructing the `Condition` struct we recursively call `get_condition` on all the items in a threshold and short-circuit if there's an error somewhere (for example, because the policy-path hasn't been provided for a specific threshold). (Alekos Filini)

Pull request description:

  Fixes #942

  ### Description

  This backports "Fix policy condition calculation" (#932) onto release/0.28

  ### Notes to the reviewers

  Currently kept the Author the same and the committer is myself. Let me know if this is not the desired outcome of the history. (I have made no modifications myself)

  ### Changelog notice

  Backported "Fix policy condition calculation"

  ### 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

ACKs for top commit:
  notmandatory:
    ACK 9cffaad71f3d090196a3e35e84ad714118ea2846

Tree-SHA512: c505bc9c8efd75cfa23db7a0e77f010f1758a1b6725661a1af1bcb40c6fd606a13e50ca005c7752fe28c3b7cd748c182fdb5870693e299adaf230adeef02517a