Merge bitcoindevkit/bdk-cli#61: Improve error handlings.

7e65add3e342747c1f0e0e0cbbf1539db30b36e4 Handle errors properly instead of unwraps. (rajarshimaitra)

Pull request description:

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  ### Description

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  bdk-cli had lots of ugly unwraps through out the code. All the unwraps inside the library (except tests) were removed and errors were propagated with `bdk::Error` type.

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  ### Checklists

  #### All Submissions:

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

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Tree-SHA512: 5c4a46af66341e300ac25339a402ea6ba9c8b81172873dc1b7ab5fa081fcbd6546fe2e372acce2a2b6f227034159724e8fc083e02c598de6a44cc4f843e1f07f