always print a message in case of an error

git-svn-id: https://vsmartcard.svn.sourceforge.net/svnroot/vsmartcard@697 96b47cad-a561-4643-ad3b-153ac7d7599c

npa/src/npa-tool.c

@@ -582,5 +582,8 @@ err:
     sc_disconnect_card(card);
     sc_release_context(ctx);
 
+    if (r < 0)
+        fprintf(stderr, "Error: %s\n", sc_strerror(r));
+
     return -r;
 }