AUTO-407 fix: Giving 400 validation error instead of 500

Since the errors are caused by croniter throwing exceptions other than ValueError, I am catching blanket exceptions and converting them to `ValueError`. Also not allowing seconds. However, following 2 issues still remain due to bug in croniter which are tracked in AUTO-475:

- '* * * * 7' works (last element is between 0 to 6)
- '* * 0 * *' works (day of the month should be 1-31)