[backport#15583] wallet: Log and ignore errors in ListWalletDir and IsBerkeleyBtree
[backport#15583] wallet: Log and ignore errors in ListWalletDir and IsBerkeleyBtree

Summary:
wallet: Log and ignore errors in ListWalletDir and IsBerkeleyBtree (João Barbosa)

Pull request description:

  Use the `noexcept` members of `boost::filesystem::recursive_directory_iterator` in order to ignore `boost::filesystem::directory_iterator::construct: Permission denied` errors. The errors are logged though.

  Steps to reproduce the issue:

  ```sh
  # 1. create directory for -walletdir without read access:
  mkdir /tmp/foo && chmod a-r /tmp/foo

  # 2. run bitcoin-qt and should print an error, but continues running:
  /Volumes/Bitcoin-Core/Bitcoin-Qt.app/Contents/MacOS/Bitcoin-Qt -regtest -walletdir=/tmp/foo
  /private/tmp/foo: Permission denied

  # 4. go to File -> Open Wallet and should segfault:
  EXCEPTION: N5boost10filesystem16filesystem_errorE
  boost::filesystem::directory_iterator::construct: Permission denied: "/private/tmp/foo"
  bitcoin in Runaway exception
  ```

---

Backport of Core [[https://github.com/bitcoin/bitcoin/pull/15583 | PR15583]]

Test Plan:
  ninja check check-functional

run bitcoin-qt with a read-only -walletdir, do not segfault when hovering over 'create wallet'

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7350

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