Fix duplicate error printing in ConfigTree

Dmitri Naumovrequested to merge
endJunction/ogs:CleanupConfigTreeDuplicateErrors into master

When assertNoSwallowedErrors() is called second time in ogs.cpp (or in ogs_python_module.cpp) the errors are printed second time. Solution to this is to clear the error messages list after they have been printed.

Fixes #3593 (closed)

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