append to instead of overwrite the initial message

```
/home/c/Desktop/mruby-git/src/hash.c:21:8: error: 4 Actual: 5r of arguments
      passed to variadic mruby C API. Expected:
  h2 = mrb_funcall(mrb, key, "hash", 0, 0);
       ^
1 error generated.
```
Should be:
```
/home/c/Desktop/mruby-git/src/hash.c:21:8: error: Wrong number of arguments
      passed to variadic mruby C API. Expected: 4 Actual: 5
  h2 = mrb_funcall(mrb, key, "hash", 0, 0);
       ^
1 error generated.
```