Merge bitcoin/bitcoin#26005: Wallet: Fix error handling (copy_file failure in RestoreWallet, and in general via interfaces)

c3e536555aa3a7db773170671da1256a2ace2094 Bugfix: Wallet: Return util::Error rather than non-error nullptr when CreateWallet/LoadWallet/RestoreWallet fail (Luke Dashjr)
335ff98c8a64eda38a2a2334102bd253f108c253 Bugfix: Wallet: Wrap RestoreWallet content in a try block to ensure exceptions become returned errors and incomplete wallet directory is removed (Luke Dashjr)

Pull request description:

  Bug 1: `copy_file` can throw exceptions, but `RestoreWallet` is expected to return a nullptr with a populated `errors` parameter. This is fixed by wrapping `copy_file` and `LoadWallet` (for good measure) in a `try` block, and converting any exceptions to the intended return style.

  Bug 2: `util::Result` turns what would have been a `false` unique_ptr into a `true` nullptr result, which leads to nullptr dereferences in at least the 3 cases of wallet creation/loading/restoring. This is fixed by keeping the pointer as a plain `std::unique_ptr` until actually returning it (ie, after the nullptr check).

  Fixes https://github.com/bitcoin-core/gui/issues/661

ACKs for top commit:
  achow101:
    ACK c3e536555aa3a7db773170671da1256a2ace2094

Tree-SHA512: 4291b3dbbb147acea2e63a704324c9371bc16ecb4237f8753729b0b0a6e55c9758ad61bfe8bd432fd7b0bae95d8b63a9831e61ac8b8d5c0197b550a2e0f4a105

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