Wallet migration fails silently when the target _WatchingOnly.lmdb already exists

[dkjn28]

Migrating a legacy wallet a second time fails with a message the user never sees, and the migration cannot be retried without manual filesystem surgery.

AssetWallet_Single::forkWatchingOnly() refuses to overwrite an existing file:

//check file does not exist
if (FileUtils::pathExists(newPath, 0)) {
   throw WalletException("WO wallet filename already exists");
}

CppBridge::migrateWallet() catches it and returns it as reply.setError(e.what()), but the GUI surfaces only:

Migration of Wallet <id> failed!

The exception text does not reach the log either: Armory spawns the bridge with --ephemeral, and main.cpp:63 drops the log level to LogLvlWarn in that case, so nothing is recorded.

The result is a dead end. A user whose first migration produced a watching-only wallet (see the unencrypted-wallet case) retries, gets "failed", and has no way to learn that the fix is to move armory_<id>_WatchingOnly.lmdb out of the data dir.

Two things would help independently:

• surface the error string the bridge already returns, instead of a generic "failed"
• either pick a non-colliding filename, or say explicitly which file is in the way

[goatpig]

This is part of the "merge" flow when restoring wallets. The typical scenario is this: you forgot the passphrase for your wallet so you want to change it. You restore from the paper backup and are offered the opportunity to carry over the meta data from the current wallet into the new one.

GUI has to be updated to handle this edge case.