Uninformative error when navbar structure field missing

[JosiahParry]

I am working with a new package with a DESCRIPTION file that does not have the repository or the URL.
I've added the following to my _pkgdown.yml

navbar:
  right: [github]

Upon running pkgdown::build_site() I am presented with the error

Error: 
! in callr subprocess.
Caused by error in `purrr::map2(links, seq_along(links), tackle_link, is_submenu = is_submenu, …`:
! In index: 1.
ℹ See `$stdout` for standard output.
Type .Last.error to see the more details.

This occurs because the github link cannot be found.

Ask: improve the error messaging in this context

EDIT: this is because the structure field of the yaml was missing.