Check if working dir exists

[Glutexo]

To check whether it’s empty, it would be nice UX to check and report whether it exists.

Currently, File.Error is raised if urls.txt doesn’t exist. Fixing #216 causes a nicer message to br printed instead of a traceback even if the directory does not exist. A different message would be nicer though.

[Glutexo]

This should cover a situation, when a file exists on the path that should be the working directory.