Error for missing breadcrumbs (scientific-python/blog.scientific-python.org#158) (#584)

* layouts/partials/breadcrumbs.html: Log an error and fail the build
if a breadcrumb is missing (i.e. if a page used in a breadcrumb has a
missing "title" field in its front matter).

This only applies to content pages which are referred to in
breadcrumbs; other pages are still allowed to not have titles.

See <scientific-python/blog.scientific-python.org#158>.

Author: alphapapa

layouts/partials/breadcrumbs.html

@@ -1,6 +1,11 @@
 <ul id="breadcrumbs" class="bd-breadcrumbs">
   {{- range $index, $ := .Ancestors.Reverse }}
   {{ if $index }}
+
+  {{ or .Title
+        (errorf "Missing breadcrumb: Page has no \"title\" field.  Filename:%q"
+                .File.Filename) }}
+
   <li class="breadcrumb-item"><a href="{{ .RelPermalink }}">{{ .Title }}</a></li>
   {{ else }}
   <li class="breadcrumb-item breadcrumb-home"><a href="{{ .RelPermalink }}"><i class="fas fa-home"></i></a></li>