make default value for `:message` on `AM::Errors` explicit.

the old signatures were a bit misleading:

```ruby
def add(attribute, message = nil, options = {})
def added?(attribute, message = nil, options = {})
```

I think for documentation purposes it's better to have the default value directly in the signature. While this introduces a bit of duplication it's closer to the documentation text, which also mentions the default value and would need to change.

Closes #11048