[AIRFLOW-2048] Fix task instance failure string formatting

Closes #2990 from AetherUnbound/bugfix/job-error-
collection

airflow/jobs.py

@@ -2312,7 +2312,7 @@ class BackfillJob(BaseJob):
         if ti_status.failed:
             err += (
                 "---------------------------------------------------\n"
-                "Some task instances failed:\n%s\n".format(ti_status.failed))
+                "Some task instances failed:\n{}\n".format(ti_status.failed))
         if ti_status.deadlocked:
             err += (
                 '---------------------------------------------------\n'