NetDb: If NetDb is empty, throw exception instead of killing NetDb thread

libi2pd/NetDb.cpp

@@ -2,6 +2,7 @@
 #include <fstream>
 #include <vector>
 #include <boost/asio.hpp>
+#include <stdexcept>
 
 #include "I2PEndian.h"
 #include "Base.h"
@@ -139,8 +140,7 @@ namespace data
 					auto numRouters = m_RouterInfos.size ();
 					if (numRouters == 0)
 					{
-						LogPrint(eLogError, "NetDb: no known routers, reseed seems to be totally failed");
-						break;
+                                                throw std::runtime_error("No known routers, reseed seems to be totally failed");
 					}
 					else // we have peers now
 						m_FloodfillBootstrap = nullptr;