MockFileSystem.Directory.GetFiles throws DirectoryNotFoundException with invalid characters in directory path

[jeremiaskoenig]

When calling MockFileSystem.Directory.GetFiles(string path, string searchPattern) with a path containing invalid path characters from MockFileSystem.Path.GetInvalidPathChars() a DirectoryNotFoundException is thrown instead of the anticipated ArgumentException which gets thrown by FileSystem.Directory.GetFiles(string path, string searchPattern) creating a different behaviour between test and regular implementation.

[robkeim]

Thanks for reporting this @jeremiaskoenig! I took a crack at fixing this in #370.