`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.
#include <assert.h>
#include <future>
#include <boost/algorithm/string/replace.hpp>
static const size_t OUTPUT_GROUP_MAX_ENTRIES = 10;
static CCriticalSection cs_wallets;
static std::vector<std::shared_ptr<CWallet>> vpwallets GUARDED_BY(cs_wallets);
bool AddWallet(const std::shared_ptr<CWallet>& wallet)
{
    LOCK(cs_wallets);
    assert(wallet);
    std::vector<std::shared_ptr<CWallet>>::const_iterator i = std::find(vpwallets.begin(), vpwallets.end(), wallet);
    if (i != vpwallets.end()) return false;
    vpwallets.push_back(wallet);
    return true;
}
bool RemoveWallet(const std::shared_ptr<CWallet>& wallet)
{
    LOCK(cs_wallets);
    assert(wallet);
    std::vector<std::shared_ptr<CWallet>>::iterator i = std::find(vpwallets.begin(), vpwallets.end(), wallet);
    if (i == vpwallets.end()) return false;
    vpwallets.erase(i);
    return true;
}
bool HasWallets()
{
    LOCK(cs_wallets);
    return !vpwallets.empty();
}
std::vector<std::shared_ptr<CWallet>> GetWallets()