Allow to modify the "There are no shipping rates available for this address." error message

Done! Please review!

Thanks @grevil, looks good to me. This goes one step further instead of only making the message configurable.

So the maintainers should decide, if this is fine for them (while I think it makes sense for more flexibility and styling) or if only the message text should be configurable.

Automatically closed - issue fixed for 2 weeks with no activity.