When migrating any legacy unnamed wallet, a failed migration would
cause the cleanup logic to remove its parent directory. Since this
type of legacy wallet lives directly in the main '/wallets/' folder,
this resulted in unintentionally erasing all wallets, including the
backup file.

To be fully safe, we will no longer call `fs::remove_all`. Instead,
we only erase the individual db files we have created, leaving
everything else intact. The created wallets parent directories are
erased only if they are empty.
As part of this last change, `RestoreWallet` was modified to allow
an existing directory as the destination, since we no longer remove
the original wallet directory (we only remove the files we created
inside it). This also fixes the restore of top-level default wallets
during failures, which were failing due to the directory existence
check that always returns true for the /wallets/ directory.

This bug started after:
bitcoin@f6ee59b
Previously, the `fs::copy_file` call was failing for top-level wallets,
which prevented the `fs::remove_all` call from being reached.

Github-Pull: bitcoin#34156
Rebased-From: f4c7e28

Verifies that a failed migration of the unnamed (default) wallet
does not erase the main /wallets/ directory, and also that the
backup file exists.

Github-Pull: bitcoin#34156
Rebased-From: 36093bd

…rune failure

The first test verifies that restoring into an existing empty directory
or a directory with no .dat db files succeeds, while restoring into a
dir with a .dat file fails.

The second test covers restoring into the default unnamed wallet
(wallet.dat), which also implicitly exercises the recovery path used
after a failed migration.

The third test covers failure during restore on a prune node. When
the wallet last sync was beyond the pruning height.

Github-Pull: bitcoin#34156
Rebased-From: f011e0f

Right now, after migration the last message users see is "migration completed",
but the migration isn't actually finished yet. We still need to load the new wallets
to ensure consistency, and if that fails, the migration will be rolled back. This
can be confusing for users.

This change logs the post-migration loading step and if a wallet fails to load and
the migration will be rolled back.

Github-Pull: bitcoin#34156
Rebased-From: d70b159

Because the default wallet has no name, the watch-only and solvables
wallets created during migration end up having no name either.

This fixes it by applying the same prefix name we use for the backup
file for an unnamed default wallet.

Before: watch-only wallet named "_watchonly"
After:  watch-only wallet named "default_wallet_watchonly"

Github-Pull: bitcoin#34156
Rebased-From: 82caa81

…eight

Github-Pull: bitcoin#34156
Rebased-From: b7c34d0