feat: modify return codes

Modify exit codes. `cheat` now returns an exit code value `2` on errors
pertaining to a cheatsheet not being found.

BREAKING CHANGE

cmd/cheat/cmd_view.go

@@ -39,7 +39,7 @@ func cmdView(opts map[string]interface{}, conf config.Config) {
 	sheet, ok := consolidated[cheatsheet]
 	if !ok {
 		fmt.Printf("No cheatsheet found for '%s'.\n", cheatsheet)
-		os.Exit(0)
+		os.Exit(2)
 	}
 
 	// apply colorization if requested