fix: use invalid cron value in error msg instead of undefined e

[Andrej730]

Hello. Noticed unbound variable e in ScheduledBackupSerializer.validate() - probably didn't cause any harm since it was about to raise another error anyway, though it wouldn't be expected ValidationError.

HouseWatch/housewatch/api/backups.py

Lines 46 to 51 in ee46e99

	def validate(self, data):
	if data.get("schedule") and not croniter.is_valid(data["schedule"]):
	raise serializers.ValidationError(f"Invalid cron expression: {e}")
	if data.get("incremental_schedule") and not croniter.is_valid(data["incremental_schedule"]):
	raise serializers.ValidationError(f"Invalid cron expression: {e}")
	return data

Checked the history, it was there from the start, when it was added in 0cb928c:

https://github.com/PostHog/HouseWatch/blame/0cb928cd55c4b767dc0a6483df14c432ee1339f6/housewatch/api/backups.py#L40-L49

Fixed it by including schedule/incremental_schedule in the error instead.

@feliperalmeida can you please take a look?