Return a status enum from ScanForWalletTransactions

Return the failed block as an out var.

This clarifies the outcome as the prior return value could
be null due to user abort or failure.

src/test/test_bitcoin.h

@@ -15,9 +15,17 @@
 #include <txmempool.h>
 
 #include <memory>
+#include <type_traits>
 
 #include <boost/thread.hpp>
 
+// Enable BOOST_CHECK_EQUAL for enum class types
+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;