Catch ImagePullBackOff from receptor workunit..

and surface to job_explanation instead of just result traceback so we stop getting the
```
Failed to JSON parse a line from worker stream. Error: Expecting value: line 1 column 1 (char 0) Line with invalid JSON data: b''
```
for this specific case

awx/main/tasks/receptor.py

@@ -537,6 +537,12 @@ def _run_internal(self, receptor_ctl):
                     self.task.update_model(self.task.instance.pk, status='pending')
                     return
 
+                if 'ImagePullBackOff' in detail:
+                    logger.warning(detail)
+                    log_name = self.task.instance.log_format
+                    logger.warning(f"Could not launch pod for {log_name}. ImagePullBackOff.")
+                    self.task.runner_callback.delay_update(job_explanation=f'{detail}')
+
                 try:
                     receptor_output = ''
                     if state_name == 'Failed' and self.task.runner_callback.event_ct == 0: