It's not that complicated. Do your due diligence and you won't land on a construction site. If this had been RYR, EZY or WZZ I'm sure our German colleagues wouldn't have been so understanding as they appear to be in this thread.
Admit the mistake, all of us should learn from it and move on. I mean, KLM also took off from a taxiway in their own home base a while ago. That stuff happens. I don't think they were trying to make the excuse that the runway wasn't marked well enough. Don't blame the Poles for building a better runway instead of that roller coaster they have there now. If you fly in somewhere and you're not familiar, do your homework.
There I go pre-empting the 'relevant facts', yet somehow this situation feels very obvious to me.