default_error_messages is an dictionary with 3 entries: return value of the function _ called with an argument,
default_error_messages = { 'list' : _ ( 'Enter a list of values.' ) , 'invalid_choice' : _ ( 'Select a valid choice. %(value)s is not one of the' ' available choices.' ) , 'invalid_pk_value' : _ ( '"%(pk)s" is not a valid value for a primary key.' ) }