Check for errors when opening program file.

Add a debug assertion in the reader to catch bugs like the last one.

reader.c

@@ -621,6 +621,7 @@ static value_t patch_placeholders(reader_state_t *state, value_t v)
 
   for (value_t item = state->ref_alist.value; !is_nil(item); item = _CDDR(item))
   {
+    assert(!is_placeholder(state, _CADR(item)));
     tree_replace(&v, item, _CADR(item));
   }
 

rosella.c

@@ -45,8 +45,11 @@ int main(int argc, char **argv)
   }
   else
   {
-    gc_root_t argv_root;
     FILE *f = fopen(argv[1], "r");
+
+    if (f)
+    {
+      gc_root_t argv_root;
       value_t program;
 
       register_gc_root(&argv_root, NIL);
@@ -64,6 +67,11 @@ int main(int argc, char **argv)
       fclose(f);
       unregister_gc_root(&argv_root);
     }
+    else
+    {
+      perror(argv[1]);
+    }
+  }
 
   return 0;
 }