multi: don't pass err.Error() to Fatalf

It resulted in interpreting the error message as a format string.
Use Fatal(err) instead.

contractcourt/breach_arbitrator_test.go

@@ -2070,7 +2070,7 @@ func assertBrarCleanup(t *testing.T, brar *BreachArbitrator,
 
 	}, 5*time.Second)
 	if err != nil {
-		t.Fatalf(err.Error())
+		t.Fatal(err)
 	}
 }