Werk #2548: Fixed exceptions in different places in case of user errors

In different places, e.g. during login errors it could happen that exceptions like this occur:

AttributeError: MKUserError instance has no attribute 'message'

It seems that this occurance depends on the used Python version.

Internal note: Never use e.message to get the text of an exception. Better directly put the exception into a string format: "%s" % e

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