Warn user if task selected to run was not found (#160)

* Warn user if task selected to run was not found

* correct mistakes made by merge

* fix flake8 `admin.py:219:1: W293 blank line contains whitespace`

Author: iAnanich

django_celery_beat/admin.py

@@ -3,7 +3,7 @@
 
 from django import forms
 from django.conf import settings
-from django.contrib import admin
+from django.contrib import admin, messages
 from django.db.models import When, Value, Case
 from django.forms.widgets import Select
 from django.template.defaultfilters import pluralize
@@ -202,6 +202,21 @@ def run_tasks(self, request, queryset):
                   task.queue)
                  for task in queryset]
 
+        if any(t[0] is None for t in tasks):
+            for i, t in enumerate(tasks):
+                if t[0] is None:
+                    break
+
+            # variable "i" will be set because list "tasks" is not empty
+            not_found_task_name = queryset[i].task
+
+            self.message_user(
+                request,
+                _('task "{0}" not found'.format(not_found_task_name)),
+                level=messages.ERROR,
+            )
+            return
+
         task_ids = [task.apply_async(args=args, kwargs=kwargs, queue=queue)
                     if queue and len(queue)
                     else task.apply_async(args=args, kwargs=kwargs)