DRY: refactor request termination code into its own function.

The code to terminate a request (echo a message, add a status code header, call drupal_page_footer() and exit) is repeated numerous times throughout restws.module. Attached is a patch that aims to get rid of the duplicated code and improve legibility by refactoring it into a restws_terminate_request() function (I would have preferred to name it restws_exit(), analogous with drupal_exit(), but this makes Drupal think it is an hook_exit implementation).