`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.
/**
     * keystore implementation
     * Generate a new key
     */
    CPubKey GenerateNewKey(WalletBatch& batch, bool internal = false) EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
    
    bool AddKeyPubKey(const CKey& key, const CPubKey &pubkey) override EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
    bool AddKeyPubKeyWithDB(WalletBatch &batch,const CKey& key, const CPubKey &pubkey) EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
    
    bool LoadKey(const CKey& key, const CPubKey &pubkey) { return CCryptoKeyStore::AddKeyPubKey(key, pubkey); }
    
    void LoadKeyMetadata(const CKeyID& keyID, const CKeyMetadata &metadata) EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
    void LoadScriptMetadata(const CScriptID& script_id, const CKeyMetadata &metadata) EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
    bool LoadMinVersion(int nVersion) EXCLUSIVE_LOCKS_REQUIRED(cs_wallet) { AssertLockHeld(cs_wallet); nWalletVersion = nVersion; nWalletMaxVersion = std::max(nWalletMaxVersion, nVersion); return true; }
    void UpdateTimeFirstKey(int64_t nCreateTime) EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
    
    bool AddCryptedKey(const CPubKey &vchPubKey, const std::vector<unsigned char> &vchCryptedSecret) override;
    
    bool LoadCryptedKey(const CPubKey &vchPubKey, const std::vector<unsigned char> &vchCryptedSecret);
    bool AddCScript(const CScript& redeemScript) override;
    bool LoadCScript(const CScript& redeemScript);
    
    bool AddDestData(const CTxDestination &dest, const std::string &key, const std::string &value);
    
    bool EraseDestData(const CTxDestination &dest, const std::string &key);
    
    void LoadDestData(const CTxDestination &dest, const std::string &key, const std::string &value);
    
    bool GetDestData(const CTxDestination &dest, const std::string &key, std::string *value) const;