Fix crash on inaccessible directory

[michaeladler]

qmarkdown crashes if a directory with inaccessible permissions (e.g. owner root, mode 0700) is encountered while scanning the
workspace. The crash is caused by an exception which is thrown by the fs::directory_iterator.

This is similar to the fix I have done here, but unfortunately a little less pretty in the case of auto_complete where we don't need any recursion: the legacy API std::experimental::filesystem::directory_iterator::directory_iterator does not provide the option to set directory_options in the second argument, so I had to switch to the recursive version and bail out on depth > 0. In the case of resgen.cpp it removes the need for manual recursion.

[Peterwmoss]

Thanks for taking a look at this - the auto complete is still very much WIP, but I don't have a lot of free time to work on this project at the moment.

I'll take a look at it in the morning when I wake up as I now have somewhat of a vacation, but initial look tells me that it's good 😄

[Peterwmoss]

Looks good - thanks for your contribution! I'll merge when checks pass.