Supress exception with concurrent deletion

Relates https://github.com/ansible/ansible-tower/issues/7768

This issue, as well as
https://github.com/ansible/ansible-tower/issues/7622, both rooted in a
concurrency issue of Django ORM:
https://github.com/ansible/ansible-tower/issues/762://code.djangoproject.com/ticket/28806

The solution related deals specifically with the related issue, but is
not a general solution. A general workaround can be found in
https://github.com/ansible/tower/pull/500.

Signed-off-by: Aaron Tan <jangsutsr@gmail.com>

awx/main/access.py

@@ -12,6 +12,7 @@ from django.db.models import Q, Prefetch
 from django.contrib.auth.models import User
 from django.contrib.contenttypes.models import ContentType
 from django.utils.translation import ugettext_lazy as _
+from django.core.exceptions import ObjectDoesNotExist
 
 # Django REST Framework
 from rest_framework.exceptions import ParseError, PermissionDenied, ValidationError
@@ -1134,8 +1135,11 @@ class ProjectUpdateAccess(BaseAccess):
 
     def can_start(self, obj, validate_license=True):
         # for relaunching
-        if obj and obj.project:
-            return self.user in obj.project.update_role
+        try:
+            if obj and obj.project:
+                return self.user in obj.project.update_role
+        except ObjectDoesNotExist:
+            pass
         return False
 
     @check_superuser