Fields with errors

[dankimio]

Bootstrap provides helper classes for inputs with errors like .has-error or .has-warning. Rails wraps all fields with errors in blocks with .field_with_errors class. There're many options:

• Extend .field_with_errors class with Bootstrap .has-danger class
• Provide an initializer via generator that will add .has-danger class to blocks with errors in config.action_view.field_error_proc
• Add the same configuration option but in a Railtie initializer