init: If bitcoind is unreachable, explain where the user can get it

Fixes: https://github.com/greenaddress/elements/issues/2

src/init.cpp

@@ -1874,6 +1874,8 @@ bool AppInitMain()
     uiInterface.InitMessage(_("Awaiting mainchain RPC warmup"));
     if (!MainchainRPCCheck(true)) { //Initial check, fail immediately
         return InitError(_("ERROR: elementsd is set to verify pegins but cannot get valid response from the mainchain daemon. Please check debug.log for more information."));
+        + "\n\n"
+        + strprintf(_("If you haven't setup a %s please get the latest stable version from %s or if you do not need to validate pegins set in your elements configuration %s"), "bitcoind", "https://bitcoincore.org/en/download/", "validatepegin=0"));
     }
 
     uiInterface.InitMessage(_("Done loading"));