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

```
Missing helper file helpers/my_helper.rb
```

Now when exception does not apply required helper rails will throw
proper exception:

```
No such file to load -- missing
```