Various improvements

[jb-abbadie]

No description provided.

[Vince-Chenal]

Just a little suggestion but OK for me

[Vince-Chenal]

I would prefer it written this way to remove indentation levels

for _, element := range versions {
	if !constrains.Check(element) { // Validate a version against a constraint
		continue
	}

	tfversion := element.String()
	if !lib.ValidVersionFormat(tfversion) { // check if version format is correct
		printInvalidTFVersion()
		os.Exit(1)
	}
	
	out, err := lib.Install(tfversion, mirrorURL)
	if err != nil {
		log.Printf("Error during install %v", err)
		os.Exit(1)
	}
	return out
}