ndnsec: improve error reporting when loading a Certificate or SafeBag fails

Refs: #5164
Change-Id: I6f594a921bb063ad808f311d8ff978bf0f7d528d
diff --git a/tools/ndnsec/import.cpp b/tools/ndnsec/import.cpp
index 7e27bee..2fb6755 100644
--- a/tools/ndnsec/import.cpp
+++ b/tools/ndnsec/import.cpp
@@ -23,7 +23,6 @@
 #include "util.hpp"
 
 #include "ndn-cxx/security/impl/openssl.hpp"
-#include "ndn-cxx/util/io.hpp"
 #include "ndn-cxx/util/scope.hpp"
 
 namespace ndn {
@@ -74,11 +73,7 @@
 
   KeyChain keyChain;
 
-  shared_ptr<security::SafeBag> safeBag;
-  if (input == "-")
-    safeBag = io::load<security::SafeBag>(std::cin);
-  else
-    safeBag = io::load<security::SafeBag>(input);
+  auto safeBag = loadFromFile<security::SafeBag>(input);
 
   if (password.empty()) {
     int count = 3;
@@ -91,7 +86,7 @@
     }
   }
 
-  keyChain.importSafeBag(*safeBag, password.data(), password.size());
+  keyChain.importSafeBag(safeBag, password.data(), password.size());
 
   return 0;
 }