Small Improvement: Aggregate Data dont allow wrong call

[timo-at-lanalabs]

df = api.aggregate(log_id=log_id, metric="frequency",grouping= "byActivity")
This receives an error like: 400 Client Error: Bad Request for url: http://localhost:4000/api/v2/aggregate-data

This is because values_from= "allEvents" is missing. We should not allow this request to go through and send a warning to the user.