Search for on old/defect copy of exercism in PATH before installing

[NobbZ]

Today in the gitter there was a student who already used exercism CLI 2.x and came back after more than a year of pausing.

The student tried to use the built-in upgrade command of the CLI (it is so far unknown if the student did a manual installation, used choco or even an older version of the installer) which failed due to exercism/cli#645.

The installer should be able to recognize if there is already a file called exercism.exe available in PATH and offer to repair the installation in the old location.

As an addition it would even recognize a choco bin-stub and tell the student to uninstall using choco first before proceeding with the installer.