Add default "error" and "required" css classes to form rows

[Haisheng HU]

Ticket #3512 talked about this feature a few years ago. User can now add error_css_class and required_css_class to its own Form/ModelForm subclass. But it should be more convenient/straightforward to have these attributes assigned in its parent BaseForm.

The default output of a required/error row will be as follows.

<tr class="required error">
<th><label for="id_name">Name:</label></th>
<td>
<ul class="errorlist"><li>This field is required.</li></ul>
<input id="id_name" type="text" name="name" maxlength="256" />
</td>
</tr>

[Haisheng HU]

add error_css_class and required_css_class to BaseForm's init method

[Luke Plant]

I'm going to WONTFIX, because:

1. this isn't a very common need, and you can already achieve what you need by subclassing.

2. there are no other HTML-related attributes passed to __init__. It doesn't make sense to complicate the constructor of Form for this requirement.