ebnm_check_fn() should check that function can handle x = c(-10, 0, 10)

this is the test value of `x` used by `flashier`. If it cannot handle it then a message should be printed with a suggested solution, e.g., include

```
if (length(x) == 3){ ### just to satisfy check of custom function
    return (ebnm_flat(x))
  }
```