Still some t() missing in form_set_error instances

Further to http://drupal.org/node/360649 , there are still some instances where t() is not present in form_set_error() . This means that the tokens are not replaced, and e.g. error messages read literally:

There is already a @type_name with that @type_title. Please enter a different @type_name

Attached a patch for a couple more t() functions. I have also reworded the error message slightly, because "There is already a @type_name" is not good English if @type_name begins with a vowel e.g. "There is already a event".