Merge bitcoindevkit/bdk#582: Expose bip39::Error

c752ccbddeaa5ae441cc74a1386c1e412a447104 Expose bip39::Error (志宇)

Pull request description:

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  ### Description

  Expose `bip39::Error` (fixes #581 )

  ### Notes to the reviewers

  I am aware that the `bip39` module plans to be rewritten (as per #561 ), however this seems like a rather straightforward and quick change that may be useful in the short/mid term.

  ### Checklists

  #### Bugfixes:

  * [x] Expose `bip39::Error`

ACKs for top commit:
  afilini:
    ACK c752ccbddeaa5ae441cc74a1386c1e412a447104

Tree-SHA512: 98b7ac1ba88aed07d9160830ee80496c32d531c15ada0e9b50a97f0883fbfced22fa83a7c7f8366aadb7e7a667d8a63dde869d31cc375206d277e55b2ec3089d

diff --cc src/keys/bip39.rs
Simple merge