raise OptionNotFound when filling a select with a text for which there is no option.

When filling a select with a text for which there is no option, I get:
```ruby
NoMethodError: undefined method `select_option' for nil:NilClass
```

With this change it will raise a `OptionNotFound` exception, e.g.:
```ruby
CornerStones::Form::OptionNotFound: The select "author" does not contain an option "Zaphod Beeblebrox".
Available options: ["Robert C. Martin", "Eric Evans", "Kent Beck"]
```