unify exception handling of Beremiz and PLCOpenEditor

fix problem running PLCOpenEditor

$ ./PLCOpenEditor.py
../src/common/stdpbase.cpp(62): assert "traits" failed in Get(): create wxApp before calling this

Regression was introduced by '3311eea' (clean-up: fix PEP8 E402 module level import not at top of file)

don't check any particular wx version

Beremiz works with any version of wx.