#301 XLS Data Reader provide user with error message if ID is not specified

[guenthert]

If the XLS Data Reader is executed without a specified ID, e. g. because this is not available in the input Excel table, the XLS Data Reader create an empty data table, but the user does not know why this happens.

If would be better if the XLS Data Reader e.g. then create automatically an ID or at least gives a helpful warning.

[Christian Thöns]

Now there is a warning if the user does not assign ID in the dialog

[Matthias Filter]

Works fine now, great!!