Gracefully fail when removing exercises folder

If for whatever reason the `exercises` folder cannot be removed the
`setup.ps1` script did not stop but continue running potentionally
spilling a lot of error messages.
With this change the failing deletion will stop the setup script.

Resolves: eficode-academy#243

utils/make-exercise-repo.ps1

@@ -1,6 +1,6 @@
 # First cleanup if there is an old exercise repository
 if (Test-Path .\exercise) {
- Remove-Item .\exercise\ -force -recurse
+ Remove-Item .\exercise\ -Force -Recurse -ErrorAction:Stop
 }
 
 # Initialize a new repository