Try-catch GetDirectories() call in PathExplorer#ExploreFolder()

Reported [here](http://stackoverflow.com/questions/26084448/haxe-crashes-flashdevelop-permissions-error), and I also just ran into it myself (with a different path) while working with SourceTree.
Yes I was a bit unsure about this one - it's an error that might need to be brought to the user's attention.
Proper logging would be more lik: `TraceManager.AddAsync("Failed to explore folder X");`