Improve error message if src or test directory does not exist

[LeoVie]

Given your --src directory is Classes, but you accidentally type Classses instead.
Then you receive a confusing error message Directory name must not be empty. thrown by Symfony\Component\Finder\Iterator\RecursiveDirectoryIterator.
It would be nice if this exception could get catched and the user could get an error message like The given src directory does not exist.. Maybe it would be the best to validate the given --src and --test directories as the very first action of the RunCommand