main: Print "error: " prefix on errors

It's just clearer that something went wrong.

src/ostree/main.c

@@ -71,7 +71,7 @@ main (int    argc,
 
   if (error != NULL)
     {
-      g_printerr ("%s\n", error->message);
+      g_printerr ("error: %s\n", error->message);
       g_error_free (error);
     }