generate_help reuses the existing get_error_messages method

I needed to monkey-patch the FormBuilder and found that the message list is built the same way in two places. This patch simply reuses the existing method to get the list of error messages. 

Also, I found an issue with specifying non-default label column span on the form tag, e.g.

```
= bootstrap_form_for item, layout: :horizontal, label_col: 'col-sm-3', control_col: 'col-sm-9' do |f|
   = f.form_group do
      p Hello there
```
The paragraph would only be offset by two columns instead of three as specified at the form level.