Optionally show a Drupal message on redirects

Since redirects are often deliberately done to take a user away from a URL, it may be helpful to set a Drupal message for the user to let them know why they are on a different page to the one they originally went to.

This would just need an additional option alongside path/query/fragment (but still with replaceable 'tokens'), and a drupal_set_message just before the drupal_goto.