PyJobTransforms.trfAMI: fix handling of pyAMI.exception.Error exceptions

While handling pyAMI.exception.Error exceptions, the trfAMI.py module was attempting to access the message attribute of these exceptions in order to check the associated error message, but this results in AttributeError: 'Error' object has no attribute 'message'. In such cases str(e) should be used to access the message string.