make the api job creation errors a bit more human-friendly

(do not display the exception type)

django/allgo/api/v1/views.py

@@ -76,7 +76,7 @@ def jobs(request):
     try:
         job.save()
     except ValidationError as e:
-        return JsonResponse({'error': "Invalid parameters: %s" % e.error_dict}, status=400)
+        return JsonResponse({'error': "Invalid parameters: %s" % e.message_dict}, status=400)
 
     # FIXME: possible infoleak: because we have ATOMIC_REQUESTS=True the
     # current id can be reused for another job in case anything fails before