Show separate error for when user tries to cancel letter

that is already cancelled vs when it is too late to
cancel letter vs when we don't know what's the cause
of failure.

This is so we could show useful error messages to the users
and also for better debugging.

app/service/rest.py

@@ -2,7 +2,10 @@ import itertools
 from datetime import datetime
 
 from flask import Blueprint, current_app, jsonify, request
-from notifications_utils.letter_timings import letter_can_be_cancelled
+from notifications_utils.letter_timings import (
+    letter_can_be_cancelled,
+    too_late_to_cancel_letter,
+)
 from notifications_utils.timezones import convert_utc_to_bst
 from sqlalchemy.exc import IntegrityError
 from sqlalchemy.orm.exc import NoResultFound
@@ -473,9 +476,17 @@ def cancel_notification_for_service(service_id, notification_id):
         raise InvalidRequest('Notification cannot be cancelled - only letters can be cancelled', status_code=400)
     elif not letter_can_be_cancelled(notification.status, notification.created_at):
         print_day = letter_print_day(notification.created_at)
-
+        if too_late_to_cancel_letter(notification.created_at):
+            message = "It’s too late to cancel this letter. Printing started {} at 5.30pm".format(print_day)
+        elif notification.status == 'cancelled':
+            message = "This letter has already been cancelled."
+        else:
+            message = (
+                f"We could not cancel this letter. "
+                f"Letter status: {notification.status}, created_at: {notification.created_at}"
+            )
         raise InvalidRequest(
-            "It’s too late to cancel this letter. Printing started {} at 5.30pm".format(print_day),
+            message,
             status_code=400)
 
     updated_notification = notifications_dao.update_notification_status_by_id(