Test if file is read-only

[Drinausaur]

If we try to use the plugin on a read-only file ( like the KiCad demo files ), we get a python error.
I think it would be nicer to test if it opens properly for writing, and if not, display a user-friendly message instead of the python trace

[MitjaNemec]

I'll be a bit more specific. The issue is that you've opened a project in a read-only folder. And as the plugin places the .log file there, this causes the issue. I'll get on it