`ScanForWalletTransactions()` has the following comment about its return value:

```
 * Returns null if scan was successful. Otherwise, if a complete rescan was not
 * possible (due to pruning or corruption), returns pointer to the most recent
 * block that could not be scanned.
```

This is not strictly true. If `abortrescan` is called, then `ScanForWalletTransactions()` will return `nullptr`, even though the scan wasn't successful to the tip.

This in turn causes a minor bug in `importmulti`, where the call doesn't return the correct error and warning about potentially not finding transactions if `abortrescan` was called.

I think the correct fix is for `ScanForWalletTransactions()` to inform the caller that the rescan was aborted.
* timestamp, and which will reset the wallet's nTimeFirstKey value to 1 if
     * the watch key did not previously have a timestamp associated with it.
     * Because this is an inherited virtual method, it is accessible despite
     * being marked private, but it is marked private anyway to encourage use
     * of the other AddWatchOnly which accepts a timestamp and sets
     * nTimeFirstKey more intelligently for more efficient rescans.
     */
    bool AddWatchOnly(const CScript& dest) override EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
    
    interfaces::Chain& m_chain;
    
    WalletLocation m_location;
    
    std::unique_ptr<WalletDatabase> database;
    /**
     * The following is used to keep track of how far behind the wallet is
     * from the chain sync, and to allow clients to block on us being caught up.
     *
     * Note that this is *not* how far we've processed, we may need some rescan
     * to have seen all transactions in the chain, but is only used to track
     * live BlockConnected callbacks.
     *
     * Protected by cs_main (see BlockUntilSyncedToCurrentChain)
     */
    const CBlockIndex* m_last_block_processed = nullptr;
public:
    /*
     * Main wallet lock.
     * This lock protects all the fields added by CWallet.
     */