Avoid crash when can not get human readable description

Catch errors from cron-descriptor and display crontab as it is.
Apparently there are crontabs which work in Celery, but cron-descriptor
fails to format them.

Issue #647

django_celery_beat/models.py

@@ -8,7 +8,8 @@
 
 import timezone_field
 from celery import current_app, schedules
-from cron_descriptor import get_description
+from cron_descriptor import (FormatException, MissingFieldException,
+                             WrongArgumentException, get_description)
 from django.conf import settings
 from django.core.exceptions import MultipleObjectsReturned, ValidationError
 from django.core.validators import MaxValueValidator, MinValueValidator
@@ -316,11 +317,19 @@ class Meta:
 
     @property
     def human_readable(self):
-        human_readable = get_description('{} {} {} {} {}'.format(
+        cron_expression = '{} {} {} {} {}'.format(
             cronexp(self.minute), cronexp(self.hour),
             cronexp(self.day_of_month), cronexp(self.month_of_year),
             cronexp(self.day_of_week)
-        ))
+        )
+        try:
+            human_readable = get_description(cron_expression)
+        except (
+            MissingFieldException,
+            FormatException,
+            WrongArgumentException
+        ):
+            return f'{cron_expression} {str(self.timezone)}'
         return f'{human_readable} {str(self.timezone)}'
 
     def __str__(self):

t/unit/test_models.py

@@ -207,3 +207,40 @@ def test_trigger_update_when_deleted(self):
             'The `PeriodicTasks.last_update` has not be update.'
         )
         # Check the `PeriodicTasks` does be updated.
+
+
+class HumanReadableTestCase(TestCase):
+    def test_good(self):
+        """Valid crontab display."""
+        cron = CrontabSchedule.objects.create(
+            hour="2",
+            minute="0",
+            day_of_week="mon",
+        )
+        self.assertNotEqual(
+            cron.human_readable, "0 2 * * mon UTC"
+        )
+
+    def test_invalid(self):
+        """Invalid crontab display."""
+        cron = CrontabSchedule.objects.create(
+            hour="2",
+            minute="0",
+            day_of_week="monxxx",
+        )
+        self.assertEqual(
+            cron.human_readable, "0 2 * * monxxx UTC"
+        )
+
+    def test_long_name(self):
+        """Long day name display."""
+        # TODO: this should eventually work, but probably needs conversion
+        # before passing data to cron_description
+        cron = CrontabSchedule.objects.create(
+            hour="2",
+            minute="0",
+            day_of_week="monday",
+        )
+        self.assertEqual(
+            cron.human_readable, "0 2 * * monday UTC"
+        )