[glutenfree-equipment-train-stop] display which items are missing via floating text

[Quezler]

Currently the error message in it's equipmentgrid.lua only tells you that something is missing, not what.

Have it display what is missing (and maybe how much it still needs) using rich text, here is an example:
https://github.com/Quezler/factorio__mod--x-stacks-ltn-combinator-helper/blob/main/src/scripts/constant-combinator.lua#L58#L73

The logistic_network_has_contents may be modified so it can return multiple things, e.g. a success boolean & what is missing.