Show real LoadError on helpers require

When helper try to require missing file rails will throw exception about
missing helper.

```ruby
# app/helpers/my_helper.rb
require 'missing'

module MyHelper
end
```

And when we try do load helper

```ruby
class ApplicationController
  helper :my
end
```

Rails will throw exception. This is wrong because there is a helper file.

```
Missing helper file helpers/my_helper.rb
```

Now when helper try to require non-existed file rails will throw proper exception.

```
No such file to load -- missing
```