graceful handling of null project and inventory in workflow

awx/main/models/jobs.py

@@ -606,6 +606,12 @@ class Job(UnifiedJob, JobOptions, SurveyJobMixin, JobNotificationMixin):
         evars.update(extra_vars)
         self.update_fields(extra_vars=json.dumps(evars))
 
+    def signal_start(self, **kwargs):
+        # Block cases that would cause the task manager to error
+        if self.inventory_id is None or self.project_id is None:
+            return False
+        return super(Job, self).signal_start(**kwargs)
+
     def display_artifacts(self):
         '''
         Hides artifacts if they are marked as no_log type artifacts.

awx/main/scheduler/__init__.py

@@ -10,6 +10,7 @@ from sets import Set
 from django.conf import settings
 from django.db import transaction, connection
 from django.db.utils import DatabaseError
+from django.utils.translation import ugettext_lazy as _
 
 # AWX
 from awx.main.models import * # noqa
@@ -123,7 +124,8 @@ class TaskManager():
                 can_start = job.signal_start(**kv)
                 if not can_start:
                     job.status = 'failed'
-                    job.job_explanation = "Workflow job could not start because it was not in the right state or required manual credentials"
+                    job.job_explanation = _("Job spawned from workflow could not start because it "
+                                            "was not in the right state or required manual credentials")
                     job.save(update_fields=['status', 'job_explanation'])
                     connection.on_commit(lambda: job.websocket_emit_status('failed'))