`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.
#ifndef BITCOIN_TEST_TEST_BITCOIN_H
#define BITCOIN_TEST_TEST_BITCOIN_H
#include <chainparamsbase.h>
#include <fs.h>
#include <key.h>
#include <pubkey.h>
#include <random.h>
#include <scheduler.h>
#include <txdb.h>
#include <txmempool.h>
#include <memory>
#include <type_traits>
#include <boost/thread.hpp>
template <typename T>
std::ostream& operator<<(typename std::enable_if<std::is_enum<T>::value, std::ostream>::type& stream, const T& e)
{
    return stream << static_cast<typename std::underlying_type<T>::type>(e);
}
extern uint256 insecure_rand_seed;
extern FastRandomContext insecure_rand_ctx;
static inline void SeedInsecureRand(bool fDeterministic = false)