MAINT: enhance output when table cannot be parsed

Summary

Previously, when there was a problem with a table that shall be parsed by the crawler (e.g. wrong datatype, bad format of a date and so on) there was simply the statement: 'Cannot parse FILENAME'

Now the message that is contained in the ValueError is actually printed (not only in debugging logs).

Also, the check_datatypes method was missing an appropriate call to logging.warning.

Test Environment

Load a table using the webinterface where the table causes a ValueError in one of the converter functions.

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