[Crash-catcher] User interface text updates

Radek Biba suggested that we should really check our strings for mistakes, and pointed to some of the mistakes he spotted. I went through the GUI and found some more. However, more experienced English speaker would be better at this. We should probably engage someone to do this for us. Denys, you're good at English, maybe you would like to check our UI? :) The patch with my changes is attached. Karel