Generates a bootstrap error alert element containing an unordered list of error messages on the base object and optionally for a given set of named attribute. This is idea for rendering a block of error messages at the top of the form for hidden/special/virtual attributes (the Paperclip Rails plugin does this), or errors on the base model.

A hash can be used as the last set of arguments to pass HTML attributes to the `<ul>` wrapper.

@example A list of errors on the base model

  <%= semantic_form_for ... %>
    <%= f.semantic_errors %>
    ...
  <% end %>

@example A list of errors on the base and named attributes

  <%= semantic_form_for ... %>
    <%= f.semantic_errors :something_special %>
    ...
  <% end %>

@example A list of errors on the base model, with custom HTML attributes

  <%= semantic_form_for ... %>
    <%= f.semantic_errors :class => "awesome" %>
    ...
  <% end %>

@example A list of errors on the base model and named attributes, with custom HTML attributes

  <%= semantic_form_for ... %>
    <%= f.semantic_errors :something_special, :something_else, :class => "awesome", :onclick => "Awesome();" %>
    ...
  <% end %>