Asset package update (#581)

* Rewrite new function and deploy where we can minimise the chance of setting an asset type that is different to supported list - sets validation to exact supported list

* change wording

cmd/gctcli/validation.go

@@ -23,5 +23,6 @@ func validExchange(exch string) bool {
 }
 
 func validAsset(i string) bool {
-	return asset.IsValid(asset.Item(i))
+	_, err := asset.New(i)
+	return err == nil
 }