redis: throw error if block request failed

Fixes #3196. Before, we didn't throw an error, and the engine thought the
block isn't occupied. But in fact it might be that redis is still loading,
and the block does exist in the database. The result was a cheesy map.

src/database-redis.cpp

@@ -118,12 +118,21 @@ std::string Database_Redis::loadBlock(const v3s16 &pos)
 		freeReplyObject(reply);
 		return str;
 	}
-	case REDIS_REPLY_ERROR:
-		errorstream << "WARNING: loadBlock: loading block " << PP(pos)
-			<< " failed: " << reply->str << std::endl;
+	case REDIS_REPLY_ERROR: {
+		std::string errstr = reply->str;
+		freeReplyObject(reply);
+		errorstream << "loadBlock: loading block " << PP(pos)
+			<< " failed: " << errstr << std::endl;
+		throw FileNotGoodException(std::string(
+			"Redis command 'HGET %s %s' errored: ") + errstr);
+	}
 	}
+	errorstream << "loadBlock: loading block " << PP(pos)
+		<< " returned invalid reply type " << reply->type
+		<< ": " << reply->str << std::endl;
 	freeReplyObject(reply);
-	return "";
+	throw FileNotGoodException(std::string(
+		"Redis command 'HGET %s %s' gave invalid reply."));
 }
 
 bool Database_Redis::deleteBlock(const v3s16 &pos)