Use open type for error_msg result

[talex5]

This makes it easy to combine with other errors. e.g.

if cancelled then Error `Cancelled
else Fmt.error_msg "Failed"

[dbuenzli]

Thanks ! Your patch is in as https://github.com/dbuenzli/fmt/commit/7bb3caf2f6f326774058fc97d779a47ab84b466c