do not silently ignore errors on "backupwallet" RPC cmd

src/rpcwallet.cpp

@@ -1228,7 +1228,8 @@ Value backupwallet(const Array& params, bool fHelp)
             "Safely copies wallet.dat to destination, which can be a directory or a path with filename.");
 
     string strDest = params[0].get_str();
-    BackupWallet(*pwalletMain, strDest);
+    if (!BackupWallet(*pwalletMain, strDest))
+        throw JSONRPCError(RPC_WALLET_ERROR, "Error: Wallet backup failed!");
 
     return Value::null;
 }