Show a better error message when the folder does not exist

[nojaf]

When you run dotnet fantomas right now, you get the Input path is missing... message.
Is that a good message or should the help be shown instead?

Next, if you run dotnet fantomas foo where the folder foo doesn't exist, you get that same Input path is missing... message. This could be improved by No file or folder "foo" found on disk or something similar.

I recall an example where a path with Windows slashes was used on a Linux VM using the --check flag and it wasn't all that obvious that nothing happened.
The exit code is also 0 when you enter dotnet fantomas foo --check ; $LASTEXITCODE (in PowerShell).
This should be 1 as it is without the --check flag.