show missing vignettes in abort message

[zkamvar]

This fixes an abort() message for vignettes to actually reveal the vignettes that are missing as shown in the snapshot test:

      Error in `data_articles_index()`:
-      ! Vignettes missing from index: 
+      ! Vignettes missing from index: articles/nested, width

[hadley]

Thanks for working on this! Can you please add a bullet to the top of NEWS.md? It should briefly describe the change and end with (@yourname, #issuenumber).