refactor: clarify error message when Hugo fails to load a Widget Page

See #1595
See #1599

layouts/partials/widget_page.html

@@ -3,15 +3,24 @@
 
 {{/* Get widget page */}}
 {{ $page := "" }}
+{{ $headless_bundle := "" }}
 {{ if .IsHome }}
-  {{ $page = "/home" }}
+  {{ $page = "/home/index.md" }}
+  {{ $headless_bundle = site.GetPage $page }}
+  {{/* Check homepage exists */}}
+  {{ if not $headless_bundle }}
+    {{ errorf "Homepage not found at %s!" $page }}
+    {{ errorf "Add the `/home/index.md` homepage file to each language's content folder. For example, your site should have a `content/home/` folder containing `index.md` and your homepage sections, or for multi-language sites, `content/en/home/` and `content/zh/home/` etc. Refer to the 'Build Your Homepage' and 'Language' documentation at https://sourcethemes.com/academic/docs/ and the example homepage at https://github.com/gcushen/hugo-academic/tree/master/exampleSite/content/home/index.md ." }}
+  {{ end }}
 {{ else }}
   {{ $page = .File.Path }}
-{{ end }}
-{{ $headless_bundle := site.GetPage $page }}
-{{/* Check homepage exists */}}
-{{ if not $headless_bundle }}
-  {{ errorf "Homepage not found or duplicate homepages detected for a localization! Add the `home/` folder (especially `/home/index.md`) to each language's content folder. For example, your site should have a `content/home/` folder containing `index.md` and your homepage sections, or for multi-language sites, `content/en/home/` and `content/zh/home/` etc. Refer to the 'Build Your Homepage' and 'Language' documentation at https://sourcethemes.com/academic/docs/ and the example homepage at https://github.com/gcushen/hugo-academic/tree/master/exampleSite/content/home/index.md ." }}
+  {{ $headless_bundle = site.GetPage $page }}
+  {{/* Check widget page exists. */}}
+  {{ if not $headless_bundle }}
+    {{ errorf "Widget Page not found at %s!" $page }}
+    {{ errorf "View the Widget Page documentation at https://sourcethemes.com/academic/docs/managing-content/#create-a-widget-page . }}
+    {{ errorf "If the Hugo version is between 0.65 and 0.68, it may be a confirmed Hugo bug that is expected to be fixed in Hugo v0.69: https://github.com/gcushen/hugo-academic/issues/1595#issuecomment-605514973 ." }}
+  {{ end }}
 {{ end }}
 
 {{/* Load page sections */}}