[py2py3] Apply changes from pylint --py3k to files involved in #9818

[mapellidario]

Impact of the new feature

Files involved in #9818

Is your feature request related to a problem? Please describe.

#9818 was not checked against pylint --py3k. Its report is

************* Module src.python.WMCore.WMException
W:117,12: Indexing exceptions will not work on Python 3 (indexing-exception)

We should then modernize WMCore/WMException following pylint doc:

Indexing exceptions will not work on Python 3 Indexing exceptions will not work on Python 3. Use exception.args[index] instead.

Describe the solution you'd like

Simply change the way that the exception arguments are accessed.

Describe alternatives you've considered

There is not much we ccan do if we want to run the same code in python3

Additional context

Follow-up to #9818

[mapellidario]

Fixed by #10187, which superseded #10168