Add blank case for convertUnit

Assumes that it's in bytes. On my system (Arch Linux), values of 0 have
no unit printed next to them.

free/main.go

@@ -23,8 +23,10 @@ func convertUnit(stringUnit string) (jsh.Unit, error) {
 		return jsh.GB, nil
 	case "tB":
 		return jsh.TB, nil
+	case "":
+		return jsh.B, nil
 	default:
-		return 0, errors.New(fmt.Sprintln("Unknown unit %s", stringUnit))
+		return 0, errors.New(fmt.Sprintf("Unknown unit: %s\n", stringUnit))
 	}
 }