G_RESOURCE_OVERLAYS is a list of resource-path and filesystem-path pairs. Since on Windows filesystem paths use ':', this list can't be ':'-separated there. Fix that by making it ';'-separated on Windows. Make the parser error clearer (we're not looking for a slash, we're looking for an absolute path).
(cherry picked from commit 06bbfccc8e36d3919c84d5aae9048677a39a40c7)