REFAC: "bad" stations do not report which variable is missing
There is only the message that there has been an error like
2020-12-01 15:06:45,661 - INFO: remove station DESH014 because it raised an error: "not all values found in index 'variables'" [pre_processing.py:f_proc:336]
but this does not tell us anything about the reason it really failed. Introduce a check if all requested variables are available for requested station in the join package and indicate if variables are not available. This will improve the understanding why a distinct station is indicated as invalid station.