Error handling

Currently there's two methods of error handling:
 1. API_Connection_Manager::_error(), @uses last_error, @returns WP_Error
 2. API_Con_Mngr_Error

Error reporting should handle 3 situations:
 1. Alert js message to screen
 2. Recall window.opener and append err message as get var (as in login screens)
 3. die() with error message