Show more useful error message when `icons.toml` doesn't exist

[hwittenborn]

I've been quite busy with school up until recently, but I've finally started getting back into working on my project that utilizes Relm4. I noticed this issue while upgrading the relm4-icons crate, as I didn't have icons.toml set up in the root of my project.

[AaronErhardt]

Thanks!