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

@@ -45,24 +45,32 @@ int main(int argc, char **argv)
   }
   else
   {
-    gc_root_t argv_root;
     FILE *f = fopen(argv[1], "r");
-    value_t program;
 
-    register_gc_root(&argv_root, NIL);
-
-    /* Construct list backward, so that we don't have to reverse it. */
-    for (int i = argc - 1; i >= 2; --i)
+    if (f)
     {
-      value_t temp = string_to_value(argv[i]);
-      argv_root.value = cons(temp, argv_root.value);
+      gc_root_t argv_root;
+      value_t program;
+
+      register_gc_root(&argv_root, NIL);
+
+      /* Construct list backward, so that we don't have to reverse it. */
+      for (int i = argc - 1; i >= 2; --i)
+      {
+        value_t temp = string_to_value(argv[i]);
+        argv_root.value = cons(temp, argv_root.value);
+      }
+      
+      program = read_value(f);
+      print_value(run_interpreter(program, argv_root.value));
+      nl();
+      fclose(f);
+      unregister_gc_root(&argv_root);
+    }
+    else
+    {
+      perror(argv[1]);
     }
-    
-    program = read_value(f);
-    print_value(run_interpreter(program, argv_root.value));
-    nl();
-    fclose(f);
-    unregister_gc_root(&argv_root);
   }
 
   return 0;